For more than 10 years an infestation level of 10 Lygus hesperus Knight per 50 net sweeps (1 nymph equal to 2 adults) has been used as the economic threshold for this pest on cotton in California. In 1968, population levels of 5, 10, and 15 L. hesperus per 50 sweeps were maintained and compared with a check until Aug. 15. In the check, a mean of 10 L. hesperus per 50 sweeps was exceeded on 6 occasions to a high of 19.8 per 50 sweeps during the periods of peak squaring and bloom. At harvest, significantly more seed cotton per acre was produced in the check and 15-level than in the 5 and 10 levels. In 1969, the levels per 50 sweeps maintained were: 5 L. hesperus until an average of 5 blooms per 13-row-feet occurred; and 5, 10, and check, to Aug. 18. In the check during the periods of maximum square load and peak bloom the average densities of L. hesperus exceeded 10 per 50 sweeps on 8 sampling dates to a high of 34.3 per 50 sweeps. Significantly less seed cotton was produced by the check in the first picking, but it gained sufficiently in the second picking to obscure the differences. A comparison of the check plots for the 2-year study showed that in 1969 L. hesperus populations were higher, while 35-40% less blooms were produced. The results demonstrated (1) the importance of assessing L. hesperus populations in relation to the fruiting capacity of the plant, and (2) sustained infestation levels during the periods of peak square load and maximum bloom substantially in excess of 10 per 50 sweeps had little or no effect on yield and fiber quality. In both studies L. hesherus densities were regulated with toxaphene-DDT.