Behavior of monogeneans of the genus Dactylogyrus under low (2-5 degrees C) and high (28-34 degrees C) temperatures was studied. It is established, that under extreme conditions movements of monogeneans and egg laying become more intensive. After the last egg is laid, the parasites become motionless, their bodies shrink and shorten. The motionless state in which monogeneans survive unfavorable conditions known as anabiotic or resting stage. When moved from extreme to normal conditions, dactylogyrids can leave this stage, in case the influence of unfavorable factors have not led to irreversible changes in the parasite's organism. But if such changes took place, a transversal constriction is formed on the monogenean body close to the attachment disc, where the disruption of tegument occurs. Thus, appearance of the transversal constriction can be considered a death criterion for monogeneans of the genus Dactylogyrus. It was found that immature monogeneans are more resistant to the effect of extreme temperatures in comparison with egg laying parasites.