The bacteriophage of Bacillus larvae was isolated in Czechoslovakia for the first time. Most strains of B. larvae were found to contain a temperate non-virulent phage which could be detected by a cross-test. This bacteriophage is polyvalent and may become virulent, forming plaques with various strains of B. larvae. The morphology and ultrastructure of the phage are described for the first time. This bacteriophage of B. larvae has been designated BLA.