Today, biological control of plant pathogens such as nematodes has been widely studied. One of the promising biological control agents isPochonia chlamydosporiawith high potential to reduce root-knot nematode (RKN) and cyst nematode populations. In this study, the effects of four isolates of P. chlamydosporia var. chlamydosporia on root-knot nematode Meloidogyne javanica were examined under in vitro and greenhouse conditions. In the vitro experiment, the effects of fungus culture filtrates were evaluated on mortality of juveniles and egg hatching of M. javanicain in three time intervals (24, 48 and 72 hours after application) and three concentrations (100%, 10^-1 and 10^-2) as well as parasitism of nematode eggs. In the greenhouse experiment, the effect of different isolates of P. chlamydosporia var. chlamydosporia on tomato growth factor and nematode population were tested. Data were subjected to analysis of variance and the means were compared according to the LSD test. In the Lab trial, culture filtrates, in comparison to control, showed larvicidal effects and prohibited larval emergence from the eggs and all isolates colonized the nematode eggs. On the basis of statistical analysis in all the three laboratory experiments, IRAN 1131C and IRAN 504C isolates were more effective in comparison to the other 2 isolates. In the greenhouse trial, isolates suppressed nematode growth on the host plant and nematode percentage growth, in comparison to the control, decreased. The greenhouse data showed that the least tomato nematode infection was due to the isolates IRAN 504C and Hamedan.