A recently developed PCR-based assay, amplified fragment length polymorphism (AFLP), has been used for assessing phylogenetic relationships and the genetic structure of populations in many organisms including insects. Preliminary studies show that the procedure does not work well in acridid grasshoppers since it leads to very large numbers of indistinct bands, probably due to the large size of the genome. To overcome this drawback, we developed long primer sets with four selective nucleotides, and used these for the grasshoppers Chorthippus brunneus and C. jacobsi in northern Spain. This approach has allowed us to reduce the number of bands, so that we can compare homologous bands easily. Using the revised method, it appears that the degree of interspecific genetic differentiation of C. brunneus and C. jacobsi is relatively small.