Four different alleles in the nymphal Ixodes ricinus were found for the Malate Dehydrogenase enzyme (MDH). The allele frequency of the MDH genotypes in nymphal I. ricinus was observed to change during the day. Some genotypes were present in the morning, but not in the afternoon and evening. The physiological age of the nymphs was also determined. No significant correlation could be found between physiological age and genotypes. Based on the results it must be anticipated that the comparability of tick samples not only differs in quantity but also in quality for various hours, but not necessarily for various months. Therefore isoenzyme analysis of nymphs at least as regards the MDH alleles appears to be a convenient biological tool to analyse differences in host seeking behaviour.