An ecological study ofTetranychus urticaeKoch and its native predators was conducted on strawberry in southern California with populations being surveyed at weekly intervals during 1967–68. On Sept. 21 (4 weeks after planting), 98% of the plants were infested naturally with an average of 34.1 spider mites per leaflet. The population then gradually declined to 0.1 per leaflet by Nov. 21 with 4% of the leaflets infested remaining below 0.5 actives/leaflet through Jan. 23. The twospotted spider mite population started to increase in February and reached a peak of 139.3 actives/leaflet on May 21 with 100% of the leaflets infested. The population then declined abruptly to less than 0.5 mite/leaflet by June 18 and remained thus through the end of the study on June 25. The twospotted spider mite population crashed in each of the 8 study plots surveyed after reaching peaks ranging from 72.3 to 271.1 actives/leaflet with the peaks occurring from May 7–28. Seven insectan predators were present, with the dominantScolothrips sexmaculatus(Pergande) accounting for 84% of the total recorded. The predator population reached a peak of 3.64/leaflet on June 4. Fruit yield for the 4-month fruiting period (March–June) averaged 2.1 lb/plant (23.5 tons/acre). There was no consistent correlation between high spider mite populations and low fruit yield. It was concluded that the pest population crash was due primarily to physiological changes in the plants caused by high numbers of twospotted spider mites feeding thereon.