Three Turkish strains of the entomopathogenic nematodes, Steinernema carpocapsae (Adapazari), S. feltiae (Canakkale) and Heterorhabditis bacteriophora (Kirklareli), were tested in the laboratory for their virulence on the last instar larvae of the corn stalk borer, Sesamia cretica. Mortality rates of last instar S. cretica larvae were determined for each nematode species under four different temperature treatments. Accordingly, at 15 degrees C, mortalities of S. cretica exposed to S. carpocapsae, S. feltiae and H. bacteriophora were 48, 56 and 14%, respectively. These mortalities at 20 degrees C were 62, 76 and 50%; at 25 degrees C, 82, 90 and 90%; and at 30 degrees C (the highest application temperature), mortality was 82, 92 and 94%, respectively. These results show that S. carpocapsae and S. feltiae caused similar mortalities at all applied temperatures and their effectiveness increased as the temperature increased. Although H. bacteriophora caused low mortality at low temperatures, it infected and caused very high levels of mortality at relatively high temperatures.