Saudi Arabia frets that Egypt, its strongest Arab ally and a major recipient of Saudi funding, is falling under what it sees as the baleful influence of the Muslim Brotherhood.
Riyadh recalled its ambassador from Cairo at the weekend in a spat that underlines the misgivings of the robed princes who rule the world's top oil exporter and who have watched Egypt's revolution and its often chaotic aftermath with alarm.
They fear that political uncertainty in Egypt, which votes in a presidential election this month, may undermine a decades-old strategic bond between the two pro-U.S. Arab allies, a bond already shaken when Egyptians toppled their ruler last year.
"The Saudis viewed the ouster of (President) Hosni Mubarak as a very negative development," said Robert Jordan, the U.S. ambassador in Riyadh from 2001-03.
"They're concerned about the Muslim Brotherhood and the uncertainty of the leadership. And they're very sensitive at any hint that that movement could spread to Saudi or other Gulf countries."
Riyadh's recall for consultations of Ambassador Ahmed Kattan after protests outside the Saudi embassy against the arrest of an Egyptian lawyer in the kingdom may prove fleeting.
Egypt seems keen to have Kattan back, judging by government statements and reports in state-owned newspapers of Egyptians waving Saudi flags at the embassy calling for his return.
It was street protests outside the Saudi embassy last week that caused umbrage in Riyadh. Crowds were protesting at the arrest of Egyptian lawyer Ahmed El-Gezawi by Saudi authorities.
Egyptian activists said he had been detained for speaking out against ill-treatment of Egyptians in the kingdom. The Saudi authorities said he had been smuggling drugs.
Even if the diplomatic quarrel is smoothed over, it reflects the new fragility of a once-solid alliance between the most populous Arab nation and the richest.
Saudi Arabia last month agreed to grant Cairo $2.7 billion in aid - and has given no public sign so far of reconsidering this pledge - but it fears Egypt's political evolution will amplify the Brotherhood's regional clout while diminishing Saudi influence, said an Egyptian official who asked not to be named.
The Brotherhood and Saudi Arabia share Sunni Muslim values, but Riyadh regards the movement as an ideological competitor with an aggressively activist political doctrine that might destabilise allies and foment discord inside the kingdom.
"Withdrawing the ambassador was a way of reminding Egyptians that Saudi security concerns have to be respected," said Shadi Hamid, director of research at the Brookings Doha Centre.
"The Brotherhood hasn't really gone out of its way to reassure Saudi Arabia about regional security interests."
A Saudi government spokesman did not respond to requests for comment.
Permeating Saudi worries about the Muslim Brotherhood are decades of ideological rivalry.
"The Brothers offer a religious political discourse that's in competition with the Wahhabi one. It's something of a threat to the government because it enjoys a certain legitimacy by virtue of its religiosity," said Thomas Hegghammer, author of Jihad in Saudi Arabia.
Since the 18th century, the ruling Al Saud family have enjoyed a close alliance with clerics of the ultra-conservative Wahhabi school of Islam.
In the modern kingdom, the royal family has bankrolled the clergy and given them wide-ranging influence over government policy. In return, the clerics have espoused a political philosophy that demands obedience to the ruler, a notion that shaped Saudi dismay at last year's Arab revolts.
By contrast the Muslim Brotherhood has always promoted an active political role for Islam, first as a revolutionary organisation and more lately as a force in democratic politics.
Some Saudi leaders have accused the Brotherhood of inspiring the kingdom's main domestic opposition group, the Sahwa movement that in the 1990s agitated to bring democracy to Saudi Arabia.
"The Saudis are pragmatic enough to realise when things change. Now the Muslim Brotherhood are in power in Egypt. They have to re-evaluate the relationship," said Khalid al-Dakhil, a political sociology professor in Riyadh.
Mahmoud Ghozlan, the Brotherhood's spokesman, said the movement had had no contacts with Saudi Arabia over the recent dispute, which he described as "a summer cloud".
Under Mubarak, Egypt and Saudi Arabia were both staunch opponents of what they saw as Shi'ite Iran's efforts to expand its influence and destabilise the region.
They perceived Tehran's hand behind the Shi'ite Hezbollah movement's increasing power in Lebanon, Hamas's military take-over of the Gaza Strip and sectarian violence in Iraq.
Any new Brotherhood-led government in Egypt might prove less pro-Saudi, while maintaining a distance from Iran.
The question is whether the strategic, security and financial imperatives of Egypt and Saudi Arabia will force them to swallow their qualms about working with each other.
"If Egypt can't sustain its financial system there could be a power vacuum and the sort of situation that al Qaeda might exploit. The Saudis have an interest in maintaining the viability of Egypt's economy," said Jordan.
For its part, the Muslim Brotherhood knows Egypt has no credible donors that could substitute for Saudi Arabia, said Daniel Kurtzer, U.S. ambassador to Egypt from 1997-2001.
"I don't think the leadership of any of the Egyptian factions - the military, civil society, the Islamists - would want to change things. The problem is where the street takes Egyptian policy," he said.
Jamal Khashoggi, an influential Saudi commentator and former newspaper editor, said Riyadh was watching Egypt's transition.
"It's waiting for Egypt to settle and for a leadership to emerge before we start rebuilding the strategic alliance we have with them," he said.