Abstract: Entomopathogenic nematodes (EPN) possess many positive attributes as biocontrol agents. They are safe to humans and generally safe to other non-target organisms. To investigate the effects of ultraviolet radiation exposure on EPN, we studied the effects of 20 or 40 W UV-B radiation for different time lengths (30, 60, 90, or 120 min) on the virulence of EPN infective juveniles (IJs) to Galleria mellonella, including 9 strains from 4 species (Steinernema feltiae, S. kraussei, Heterorhabditis brevicaudis, and H. megidis) isolated from Gansu Province. The results showed that there were no differences among the strains in the survival rate of IJs with and without 30 min UV-B irradiation. The virulence of IJs varied among strains after exposure to 20 W ultraviolet radiation for 30 min: virulence of two strains (S. felitiae 0639C and S. feltiae 0663TD) was increased (P < 0.05), that of one strain (S. feltiae 0661YM) decreased (P < 0.05), and that of all others (H. brevicaudis 0641TY, H. megidis 0627M, S. felitiae 0639C, S. felitiae 0619HT, S. kraussei 0663HT, and S. kraussei 0657L) showed no significantly change (P > 0.05). For example, when measured at 32 h after infection, the mortality rate of host insects infected by S. felitiae 0639C increased significantly, from 13.3% to 73.3% (P < 0.05), whereas that of insects infected by S. felitiae 0661YM decreased significantly from 66.7% to zero (P < 0.05). At the same time, treating S. felitiae 0639C and 0663TD with UV affected host mortality 24 h after exposure to IJs:for S. felitiae 0639C, host mortality at 24 h was greater in the 40 W treatment than in either control or the 20 W treatment; for S. felitiae 0663TD, host mortality at 24 h was greater in the 20 W treatment than in either the control or the 40 W treatment. Ultraviolet radiation duration (30 min, 60 min, 90 min, or 120 min exposure) had no effect on the pathogenicity of S. felitiae 0663TD or S. felitiae 0639C (P > 0.05). Therefore, the following conclusions were drawn:All of the 9 tested strains of EPN isolated from Gansu Province had some tolerance to UV-B radiation, and, in particular, S. felitiae 0622L had a high tolerance to ultraviolet radiation. In addition, ultraviolet radiation had a variable impact on the pathogenicity of EPN in Gansu Province, as there were differences among different species and strains. For example, 20 W UV-B radiation for 30 min enhanced the pathogenicity of S. felitiae 0663TD, whereas it significantly weakened that of S. felitiae 0661YM. Ultraviolet radiation stimulated S. felitiae 0663TD and S. felitiae 0639C to infect and kill host insects more quickly, although the radiation intensity that optimized the 24-h mortality rate varied between the two strains. After 48 h, however, there was no significant difference in the pathogenicity of S. felitiae 0663TD and S. felitiae 0639C between the different radiation intensities (20 and 40 W). Additionally, IJs of S. felitiae 0663TD and S. felitiae 0639C both had high pathogenicity after 48-72 h, regardless of the radiation intensity (20 W or 40 W) or the duration of exposure (30, 60, 90, or 120 min). These findings will shed more light on using entomopathogenic nematodes under ultraviolet radiation stress.