Abstract The phenology of Fulcidax monstrosa (F.) on its host plant Byrsonima sericea DC. was studied from November 2000 to June 2003 at the Restinga de Jurubatiba National Park, Rio de Janeiro, Brazil. During this period, observations on the biology and behavior of the species were also conducted in the field and in laboratory. The female lays single eggs on young stems of its host plant and covers them with feces, building an excremental shell. The hatched larva remains within it and adds more of its own feces to the case as it grows. After a maximum of four months, the larva pupates within the case, which remains closed for another four months before adult emergence. Diapause seems to occur in the adult stage inside the pupal case before emergence. Fulcidax monstrosa is a univoltine species and showed a highly seasonal distribution along the years for all stages of development. Climatic factors as well as resource availability are suggested to influence the species seasonality.