ABSTRACT Three concentrations (1.25, 2.5, and 5×107 spores ml-1) (of the biocontrol fungus Purpureocillium lilacinum (strain AUMC 10620) were tested on citrus nematode Tylenchulus semipenetrans under in vitro and field conditions. Larvae and eggs were exposed to the fungal spores in vitro for 24, 48, and 72 h, and the findings were recorded at each time point. These results were compared with the application of the nematicide abamectin. Strain AUMC 10620 effectively reduced larval activity and egg hatching of T. semipenetrans under laboratory conditions. The highest concentration (5× 107 spores ml-1) of P. lilacinum, resulted in 89.01% immobility in the larvae, compared to abamectin, which resulted in 65.93% immobility after 48 h of exposure. These percentages of immobility were increased after 72 h of exposure (100 and 85.09%) when P. lilacinum at a concentration of 5×107 spores ml-1 and abamectin were used, respectively. On the other hand, the two other P. lilacinum concentrations (1.25, and 2.5×107 spores ml-1) affected the T. semipenetrans larvae to a lesser extent. The highest fungal concentration 5× 107 spores ml-1 inhibited the hatching of T. semipenetrans eggs in vitro with 71.34, 80, and 86.67% after 24, 48 and 72 h of treatment compared to the abamectin treatment which showed 76.67, 78, and 87% after the abovementioned periods, respectively. In addition, the application of P. lilacinum (5×107 spores ml-1) or abamectin under field conditions significantly (P<0.05) reduced the population of the major nematode species (T. semipenetrans, Tylenchorhynchus spp., Helicotylenchus spp., and Pratylenchus spp.) infesting citrus after one, two, and three weeks of treatment compared to the control treatment but with no significant (P>0.05) differences between the two treatments. Three weeks after the field application, the percentage of nematode reduction was significantly (P<0.05) smaller than the control treatment at concentrations of 5, 2.5, and 1.25×107 spores ml-1, respectively, by 78.42, 64.03, and 58.35%. It is evident from these results that the application of P. lilacinum strain (AUMC 10620) can be used in integrated pest management programs to control nematodes infesting citrus trees.