AbstractThe phylogenetic relationships of all seven known species of Cecidophyopsis mites (Acari: Eriophyidae) with their Ribes hosts have been inferred from ribosomal DNA sequences. This analysis found groups of closely related mites. There were four mite species which are gall-forming and monophagous viz. Cecidophyopsis ribis, C. selachodon, C. alpina and C. aurea. These mites colonise and gall Ribes nigrum, R. rubrum, R. alpinum and R. aureum, respectively. Cecidophyopsis grossulariae was found to be non gall-forming and has two hosts, Ribes grossularia and R. oxyacanthoides. Two recently studied species which are yet to be named were also observed. The first is a gall-forming mite species which colonise and gall two plant hosts, R. spicatum and R. nigrum. The other species is non gall forming and monophagous, specific for R. rubrum. The molecular phylogeny of the mites' known Ribes host plants was calculated using the equivalent ribosomal regions as the mites. The structure of the two phylogenetic trees (mites versus hosts) was clearly different, implying that the mite speciation did not closely follow speciation events in the plant hosts. Instead, the three groups of Ribes-infesting Cecidophyopsis mites have derived from a common galling ancestor millions of years ago. Each mite group has recently diversified onto different primary hosts. One group of mites has even lost the galling habit. The results have implications for the host range and durability of mite-resistance genes in cultivated Ribes.