The effectiveness of various strains of Steinernema carpocapsae and Heterorhabditis bacteriophora, on larvae of the Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae) were tested under different conditions of temperature (15, 20, 25 and 30º) and soil moisture (6.7, 10.0, 13.3 and 20.0% v/v). The capacity of entomopathogenic nematodes to penetrate fruit fly larvae was also analyzed. Temperature and soil moisture had a significant influence on the effectiveness of the different strains in the study. Mortality of C. capitata larvae at 15 and 30ºC was slightly lower than at 20 and 25º C. H. bacteriophora strains were in general more effective at 25ºC, while S. carpocapsae, caused higher mortalities at 20ºC. Soil moisture affected the nematode-induced mortality of C. capitata at the two extreme soil moistures conditions (6.7 and 20.0% v/v), whereas no differences were observed at the intermediate moistures (10.0 and 13.3% v/v). In general, isolate Az148 were found to be more effective than Az144. No interaction was observed between the study parameters. A high percentage of dead insects with nematodes were observed, especially when treated with H. bacteriophora. Surviving C. capitata were tested as adults for the presence of symbiotic bacteria by PCR using specific primers.