Primitively eusocial insect are often considered particularly useful for investigations into the processes underlying the origins of eusociality. Ceratinine bees have long been regarded as solitary, but sometimes exhibit oddly social traits for solitary species, while other species are known to be social, and social behaviour has been artificially induced in others still (Maeta & Sakagami 1995; Sakagami & Maeta 1995). Recent studies have had some success in elucidating aspects of the evolution of social behaviour in some groups using phylogenetics to infer historical changes in social behaviour (eg: Danforth 2002; Schwarz et al. 2003; Bull et al. 2003). Phylogenetic treatments of Ceratina have only recently been attempted in earnest (Terzo 2000) and behavioural data is lacking for many Ceratina species. Nonetheless, the Japanese fauna represents some of the most comprehensively studied Ceratina, and this study uses nucleotide data from mitochondrial COI and CytB and nuclear EF-1α regions to infer a phylogeny of Japanese Ceratina species. Data give good resolution for the subgeneric groups, and provide some support for the recent morphological phylogeny of Terzo (2000), but little resolution for higher relationships. Behavioural data suggest that sociality is variable within subgenera. The diversity and primitive nature of social behaviour in this group make it one of the most promising to provide insights into the process of social evolution, but more data are clearly needed.