The incidence of resistance to certain acaricides by the European red mite—Panonychus ulmi (Koch)—has increased severely during the past few seasons. The acaricides were organic phosphate compounds and such specific acaricides as Ovex and other closely related compounds, Chlorobenzilate and Kelthane. All of the observations on European red mite resistance in California have been from the field. Laboratory studies have not been conducted due to the difficulty of rearing this mite species under artificial conditions.