Foram realizados estudos bioecologicos de Phyllophaga cuyabana (Moser, 1918) (Coleoptera: Scarabaeidae). Foram realizados estudos no campo e no laboratório no Departamento de Entomologia da ESALQ/USP. No campo determinou-se o ciclo univoltino do inseto. Os ovos foram colocados predominantemente ate 10cm de profundidade em novembro e dezembro. A fase larval com três instares ocorreu desde novembro ate setembro e pode ser dividido em dois períodos: fase ativa e de diapausa, que ocorreu no terceiro instar, caracterizada pelo acumulo de lipídios totais e diminuição do metabolismo respiratório. A fase de pupa ocorreu em setembro e outubro. Os adultos fizeram revoadas noturnas em novembro. Em laboratório o período de incubação foi afetado pela temperatura, decrescendo com o aumento desta. As exigências térmicas foram de 195,9 graus dias e a temperatura base de 13,1ºC. O período de pupa foi afetado pela umidade do solo. As revoadas ocorreram durante 12 dias. A capacidade de postura foi de 8,8 ovos por fêmea. Os adultos foram mais atraídos pela luz amarela. A luz inibiu os adultos de saírem do solo / Bioecological studies with Phyllophaga cuyabana (Moser, 1918) were undertaken with the purpose of defining its biological cycle during the year and providing information for the management and/or control of this insect in soybean crops. The studies were conducted in the field, in Rhodic Hapludox soil, in Boa Esperança, western Paraná, and in the laboratory in the Department of Entomology of Escola Superior de Agricultura "Luiz de Queiroz" (ESALQ) of the University of São Paulo (USP), in the period from December 1990 to December 1991. In the field it was determined that P. cuyabana had one generation during the year. The eggs were predominantly laid to a depth of 10 cm, in the months of November and early December. The larval phase, with three instars, occurred from November to September and could be divided into two periods: a- the active phase, marked by the economic damages caused, with the population living in the region of the highest soybean root concentration, which served as their food, and b- the diapause phase, which occurred in the third instar, in the period from May to September and which was characterized by the accumulation of lipids, lower respiratory metabolism, and the population living in individual chambers, deeper in the soil. The insects also lived in these chambers in the pupal phase and the teneral period, in the months of September and October. Flights occurred in the months of November always at dusk and early night, and ended by midnight. In the laboratory the incubation period was affected by temperature, decreasing with higher temperatures. The thermal requirements of the egg phase were 195.9 degrees day and the threshold temperature was 13,1ºC. Soil humidity between 28,5 and 33,0% did not significantly alter the incubation period, however in extreme drought and humidity conditions (23.4 and 43.2%, respectively) mortality was 100%. During the embrionary period there was water absorption and the eggs gained weight and increased in size. The duration of the pupal period was affected by humidity, being shorter with higher humídity (38.7%). ln cages, the flight period lasted 12 days. Egg-laying capacity was 8.8 eggs per female, and eggs were laid mainly up to the 5th day. Adults were more attracted by yellow light, and the greatest attraction occurred from 7:00 p.m. to 9:00 p.m. Light affected insect behavior, inhibiting their coming out to soil surface