The influence of host plant association on diversification in phytophagous insects has been the subject of much discussion, but little formal analysis.  Leaf mining flies of the genus Phytomyza s.l. (including Chromatomyia; Diptera: Agromyzidae) comprise a large (640 described species) group of host specific herbivores well-suited for such analysis.  Most species can be placed in species groups defined by similar genitalic morphology and the tendency to feed on a specific host plant family.  A phylogeny estimate was obtained using over 3500 bases of DNA sequence data from over 100 species of Phytomyza and Chromatomyia and from additional species in four related genera as outgroups.  Results confirm the monophyly of Phytomyza + Chromatomyia (excluding a few aberrant species), as well as most recognized species groups, and underscore the importance of host conservatism at the plant family level.  The genus Chromatomyia is not monophyletic, with one major cluster feeding on Asteraceae and Poaceae, and another on herbaceous and woody hosts in several plant families; both are nested within Phytomyza.  Species and species groups feeding on Ranunculaceae are found in several unrelated lineages.   However, relationships at deeper nodes are not well supported.  Morphology of representative species was also examined and compared to descriptions of remaining species in the taxonomic literature to confirm monophyly of species groups and to estimate their species richness.  Divergence times estimated from the molecular phylogeny were then used to calculate diversification rates for the major lineages of Phytomyza.  Possible correlates of diversification rate in this group are discussed.