The horn fly,Haematobia irritans(L.), overwinters by diapausing as a pharate adult within the puparium. In south-central Texas the flies enter diapause in October and November. The peak in production of diapausing pharates occurs during the first 2 weeks of November, when there is an optimal combination of pupa production and diapause induction. Adult flies emerge from diapause from late February to early May. Those entering diapause later tend to emerge slightly earlier relative to the total spring emergence pattern, indicating a more intense diapause development. There is a substantial mortality due to pupa parasites of the genusSpalangia.The wasps overwintered within puparia of the host and entered quiescence coincident with onset of diapause in the fly.