The thyPIG 3 gene from Bacillus subtilis bacteriophage IG 3 was cloned in the plasmid pHV 33. Two recombinant plasmids, pISL 61 and pISL 62 carrying that gene are effective in transforming to thymine prototrophy both Escherichia coli (by complementation) and B. subtilis (by complementation and recombination). The comparison of cloned fragment containing the thyPIG 3 gene and the thyP 3 gene from phage phi 3 T, by restriction analysis and DNA hybridization, suggests a strong homology between the two. The thyPIG 3 gene was mapped in this study in the central region of the IG 3 genome.