Grape phylloxera, Daktulosphaira vitifoliae Fitch, is a parthenogenetic, aphid-like pest which attacks the roots and leaves of many Vitis L. species. Feeding on the roots of V. vinifera L. cultivars leads to decreased plant productivity and eventual death. Isolates of phylloxera have been grouped into ‘biotypes’ based on the aggressiveness of their feeding behaviour, relative reproductive rates, and abilities to cause decline on V. vinifera and the grape rootstock AXR1. The goal of our research was to determine whether these bio-type definitions were reflected in the genetic diversity among laboratory-cultured colonies of Californian phylloxera. Thirteen phylloxera isolates, encompassing biotypes A, B and non-A or -B types based on their feeding and reproduction, were analysed using the RAPD-PCR method. Results indicate that there were genetic differences among isolates and that there was as much polymorphism within biotypes as among biotypes. This is surprising in a parthenogenetic organism and leads us to suggest that there have been multiple introductions of phylloxera into California, that phylloxera are not obligately parthenogenetic, or that there are other mechanisms whereby phylloxera may evolve relatively rapidly.