Biological control ofTetranychus urticaeKoch on strawberry was investigated in southern California during 1968–69. Mass releases ofPhytoseiulus persimilisAthias-Henriot were made at the equivalent rate of 86,250 total per acre at weekly intervals for 4 weeks, starting January 7. Populations of prey and predators were followed at weekly intervals from December through June. A flash flood, resulting from a cloudburst on February 24, destroyed all but 2 replicates (1 check and 1 release) and covered all plants with water for several hours.T. urticaereached a peak population, averaging 508.1 and 484.2 mites per leaflet in the remaining check and release plots, respectively, on April 22. By May 13,T. urticaeaveraged only 5.0 per leaflet in the release plot compared with 178.6 per leaflet in the check.P. persimilisreached a peak population, averaging 22.1 mites per leaflet, in the release plot on May 6, compared with 3.0 per leaflet in the check. Native predators (primarilyScolothrips sexmaculatus(Pergande) were adversely affected by the flooding, remaining at low numbers through March 25 and reaching a peak of 0.5 predators per leaflet on May 27.