SUMMARYMutants of the Pseudomonas aeruginosa temperate phage D3 have been isolated which are characterised by a clear plaque, and a reduced frequency of lysogenisation. They have horn classified into three groups on their ability to co‐operate in mixed infection to give a lysogenisation frequency characteristic of the wild type. Genetic recombination has been demonstrated for phage D3 and a preliminary linkage map constructed. Crosses between representatives of the three groups of clear plaque mutants confirm the genetic diversity indicated by the co‐operation test.