The company, Egypt's largest dairy products and juices producer, confirmed the detention of CEO and deputy chairman Seifeldin Thabet in a disclosure to Egypt's bourse regarding a pending investigation into his father, ex-chairman Safwan Thabet, who was arrested in December.
"The company has no further information regarding the detention/investigation to disclose at this point," Juhayna said.
It said the investigation against Safwan Thabet regarded "personal accusations unrelated to the company".
Thabet's lawyer was not immediately available. A spokesperson for Juhayna said the company had no further comment.
Juhayna's shares tumbled 17% as trading opened on Sunday and were 10.55% down when the stock exchange closed at 1200 GMT.
Two security sources and a judicial source said Seifeldin Thabet was detained late on Saturday as part of investigations into alleged financing of terrorist groups.
In 2015 a judicial committee established by the government issued a decision ordering the seizure of Safwan Thabet's assets due to alleged links to the Muslim Brotherhood, which is banned and listed as a terrorist organisation in Egypt.
Security sources told Reuters in December that Juhayna's chairman was detained for alleged "financial irregularities". The Interior Ministry has not commented.
After Safwan Thabet's arrest, Juhayna appointed his son, Seifeldin, as acting chairman. It later appointed Mohamed el-Dogheim as chairman.
(Reporting by Ehab Farouk, Patrick Werr and Ahmed Mohamed Hassan; Writing by Aidan Lewis; Editing by Hugh Lawson and Elaine Hardcastle)