Israel's National Security Council said activists in Tunisia were planning attacks on Israeli or Jewish targets and increased its warning against travel there to the second-highest level.
The NSC's Anti-terrorism Bureau said that the danger was particularly great on the island of Djerba, where Israelis have gathered in the past for the Jewish festival of Lag Ba Omer, which takes place this year on May 9-10.
"In light of an updated situation assessment the Anti-terrorism Bureau has decided to upgrade the existing warning on travel to Tunisia," an NSC statement said, adding that it was raising the level to the second-highest.
"With emphasis on the likely public gathering in Djerba for Lag Ba Omer."
During the festival, pilgrims from Israel and Europe congregate at the island's Ghriba synagogue, the oldest in Africa, where they pray and light candles.
In 2002 a suicide bomber hit the site, killing 14 German tourists, five Tunisians and two French visitors.
The Jewish community in Tunisia is still one of the largest in the Arab world but its numbers have dropped from 100,000 at independence from France in 1956 to round 1,000 today.
Most have emigrated to France or Israel.
Lag Ba Omer commemorates the death of Shimon Bar Yochai, a second-century rabbinical sage.