Lasius (Acanthomyops) phylogeny is presented with attention given to novel male morphological characters, molecular data, putative hybrids, and observations of social parasitism in the field and laboratory. Male genitalia examined with scanning electron microscopy reveals variation between species never before described. Molecular data for eight species is incorporated into phylogenetic analysis for the first time. The five putative hybrids suggested by the last revision of the group (Wing 1968) are discussed, particularly the L(Ac) latipes x claviger hybrid recently collected and observed in the field. Field and laboratory observations of socially parasitic behavior of three species and the putative L(Ac) latipes x claviger are placed in the context of the species level phylogeny, allowing for inferences about the evolution of social parasitism.