SUMMARYTen variants of phosphoglucomutase were detected by starch-gel electrophoresis in extracts of the tick Ixodes ricinus. Agreement of phenotype frequencies with those predicted by the Hardy–Weinberg model indicated that the enzyme is coded by a single gene locus (Pgm) at which 10 alleles are segregating. Allelic proportions in 5 Irish tick samples indicated that both spatial and temporal genetic differentiation exist. It is suggested that this polymorphism may be of use as a marker for studying the relationships of I. ricinus populations in Europe, as well as in taxonomic studies. A hypothesis is advanced which offers a possible adaptive explanation of the very high Pgm heterozygosity in tick populations. It is suggested that if kinetic differences exist between the various enzyme variants, the polymorphism may act to maintain an array of individuals in each population with varying developmental rates and longevities. Such a genetic strategy could be one factor which determines that the host-infesting seasons are spread over several months, thus ensuring maximum feeding and reproductive rates in each population.