The reproductive fitness of a Prat ylenchus coffeae Population, collected from banana roots in North Vietnam, was studied in relation to temperature and moisture under screenhouse and field conditions. In the screenhouse, the population grew much more slowly during the cooler winter months than during the summer and an excess of soil Moisture reduced the nematode Population drastically. The results were confirmed in the field. The nematode population in the roots of the banana plants initially increased over time but was reduced during the cooler winter months and remained nearly constant during the wettest months when the field was almost continuously flooded. These observations may explain why P. coffeae populations on Musa do not increase to dramatic levels in North Vietnam, with its cool winter temperatures and periods of abundant rainfall during several months of the year.