Ixodes persulcatus were collected from plants in the suburbs of Irkutsk in 2008, 2009 and 2012. There was a large number of individuals with exoskeleton abnormalities, moreover the proportion of such abnormalities in population would increase from May to June. It has been experimentally established that individuals with exoskeleton abnormalities exhibit much lower survivability as compared to normal ones. Stability of ontogenesis, estimated using the variance of the difference (σ_d²), of ticks with normal and abnormal exoskeleton structure did not vary. It has been established that the stability of development on a coxa of limb I are higher than IV. Possible causes of high proportion of individuals with abnormalities in natural populations of ticks are discussed.