A field cage evaluation of the parasitoidEretmocerusnr.californicusas a biological control agent ofBemisia argentifoliiBellows&Perring was conducted in Southern California cotton during June through August of 1992. Percentage of parasitism was highest in early August at 61% in the high parasitoid release treatment and was significantly higher than both the low parasitoid release and control treatments. The density ofB. argentifoliipupae on leaves in the high parasitoid release treatment was approximately one-fifth of the other treatments. Seed cotton yield was significantly higher in the high parasitoid release cage in comparison with those of the low parasitoid release and control cages. We conclude thatE.nr.californicusmay be a useful control agent in an augmentative release strategy against whitefly on cotton and is deserving of large-scale, open field studies.