Reproductive diapause was induced in adults of the grass thrips,Anaphothrips obscurusMüller, by exposing larvae to short days. Diapause was maintained by holding the adults in the short-day regimen for ca. 60 days at 21°C but thereafter adults terminated diapause and initiated reproduction. Short days and 15°C retarded but did not prevent termination of diapause. Diapausing adults collected from the grass fields in March and exposed to long days in the laboratory produced substantially more macropterous progeny than similar adults exposed to short days. The proportion of macropterous to brachypterous adults increased as a result of crowded conditions on their host plant (‘Exeter’ bentgrass) during development, but temperature or the quality of food did not appreciably affect the ratio of macropterous to brachypterous morphs. Temperatures that exceeded 25°C drastically increased the rate of reproduction and undoubtedly controls population density when other requisites are optimum. No males ofA. obscurusdeveloped in these experiments.