The protein sequences of 18 class A 13-1actamases and 2 class C/3-1actamases were analyzed to produce a rooted phylogenetic tree using the DD peptidase of Streptomyces R61 as an outgroup. This tree supports the penicillin-binding proteins as the most likely candidate for the ancestoral origin of the class A and class C ~-lactamases, these proteins diverging from a common evolutionary origin close to the DD peptidase. The actinomycetes are clearly shown as the origin of the class A ~-lactamases found in other non-actinomycete species. The tree also divides the ~-lactamases from the Streptomyces into two subgroups. One subgroup is closer to the DD peptidase root. The other Streptomyces subgroup shares a common branch point with the rest of the class A/3-1actamases, showing this subgroup as the origin of the non-actinomycete class A ~3-1actamases. The non-actinomycete class A/3-1actamase phylogenetic tree suggests a spread of these /3-1actamases by horizontal transfer from the Streptomyces into the non-actinomycete grampositive bacteria and thence into the gram-negative bacteria. The phylogenetic tree of the Streptomyces class A ~3-1actamases supports the possibility that horizontal transfer of class A (3-1actamases occurred within the Streptomyces.