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The biological activity and genomic variation of five Kenyan, three Asian and two sublethal isolates of Plutella xylostella granulovirus (PlxyGV) were determined and compared in P. xylostella larvae reared on artificial diet. Optimal production of PlxyGV occlusion bodies (occluded virus, OB) was observed in larvae reared in isolation rather than communally. Incorporation of a juvenile hormone analogue in the artificial diet increased OB production seven times. The pathogenicity and infectivity of the isolates was compared using neonate P. xylostella. A single Kenyan isolate (PlxyGV-2.18) was shown to have a significantly shorter median lethal time to death (LT50) than the other nine isolates in other geographical populations of P. xylostella. No synergism was observed between the isolates in mixed inoculations of P. xylostella and a slower killing isolate predominated over a faster killing isolate in mixed inoculations. The presence and nature of “sub-lethal” infections of PlxyGV observed in stock cultures of P. xylostella collected in Kenya and England was studied. Sub-lethal infections were observed in approximately 23% of apparently healthy stock larvae from two of the stocks of P. xylostella (one from England and one from Kenya) but was absent from a third stock originating in England. It was possible to isolate OBs from apparently healthy P. xylostella larvae in the two virus positive stocks, indicating some productive infection. The sub-lethal isolates had similar pathogenicity and infectivity to the other Kenyan and Asian isolates (excluding PlxyGV-2.18). The genomes of the isolates were compared by restriction fragment length profiling and by sequencing specific regions of the viral genome.