A survey is given of the characters used for the differentiation of the species of Synergus Hartig, classified with section II of Mayr. Special attention is given to the phenology of the species, for each of which the life-cycle is schematized, with differentiation in spring- and summer-generation. Two forms, provisionally indicated A and B, could not satisfactorily be identified with any of the known species. Sp. В is a common inquiline of oak-apples; biological observations were made on the larvae. Synergus mutabilis Deitmer, 1924, is synonymized with Synergus albipes Hartig, 1841.