Foulk, 63, is charged with 35 counts of committing lewd acts on his foster son in the Long Beach area over a 12-year period starting in 1992 when the boy was 9 years old.
Opening statements before Judge James Pierce were on Wednesday and testimony was continuing this afternoon, a court spokesman said.
Foulk was arrested at the hospital in Napa on Feb. 24, 2010. He was held to answer to the charges in April.
Foulk served as executive director of Napa State Hospital from 2007 until his arrest.
The Los Angeles County District Attorney's Office said last year there were 13 potential victims but the alleged molestation crimes against them happened before 1988 and fell outside the statute of limitations.
Long Beach Superior Court is one of 40 courts in the Los Angeles County Superior Court's jurisdiction.