The raspberry pest Amphorophora idaei has several biotypes described by their abilities to overcome plant resistance genes. Bioassays of field populations have shown a strong shift towards A1 resistance-breaking biotypes since the 1960s. RFLP analysis of the rDNA cistron was used to study variation found within and between standard clones of 3 A.idaei biotypes and 29 field populations collected over 3 years (1990-91 and 1993) in the UK. Probing genomic DNA with the ribosomal DNA probe pBG 35 produced consistent differences in RFLPs between standard clones of biotypes. However, analysis of field populations gave more complex RFLP patterns that were not biotype-specific, unlike characteristic intergenic spacer (IGS) patterns reported for Schizaphis graminum biotypes. All but one sample collected from separate fields showed considerable genetic diversity within populations, attributed to alate migrations of parthenogenetic females in summer and males in autumn.