Temperature-sensitive mutants of bluetongue virus were isolated and classified in 6 genetic recombination groups. The frequency of recombination varied both within and between groups. The 4 mutagens used viz. nitrous acid, N-methyl-N-nitroso-N-nitroguanidine, proflavine and 5-fluorouracil were found to differ in their efficacy. The period of incubation required for maximum recombination was 48 h at 28 °C.