In replication-blocked crosses in phage λ,cos, λ’s packaging origin, provokes Red-mediated recombination in its neighborhood. In crosses of wild-type x mutantcos, cosstimulated crossover products can be packaged fromcos sites cloned medially in the two parents (Stahl F W, Kobayashi I and Stahl M M 1985J. Mol. Biol. 181: 199–209). We find that the complementary crossover products are not produced in equal numbers. In agreement with the model presented earlier (ibid), most of the recombinants inherit λ’ s right arm (R arm) from thecos+parent and λ’s left arm (A arm) from the parent with the mutantcos.