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Global climate change imposes a serious threat to natural populations of many species. Estimates of the effects of climate change-mediated environmental stresses are, however, often based only on their direct effects on organisms, and neglect the potential transgenerational effects. We investigated whether high temperature (i.e. an experimental heat wave) that is known to reduce performance of adult Lymnaea stagnalis snails affects their offspring through maternal effects. Specifically, we tested whether eggs and hatched juveniles are affected by maternal thermal environment, and how strong these effects are compared with direct effects of temperature on offspring. We examined the effect of maternal thermal environment (15°C versus 25°C) on per offspring investment (egg size), and the role of both maternal and offspring thermal environments (15°C versus 25°C) on hatching success and developmental time of eggs, offspring survival after hatching, and hatchling size at the age of five weeks. Exposure of mothers to high temperature increased hatching success of eggs, and also made the onset of hatching earlier. However, high maternal temperature reduced the survival and the final size of hatched juveniles. Direct effects of high temperature on offspring survival were negative (both eggs and hatchlings), but increased the developmental rate and growth of those eggs and hatchlings that survived. Interestingly, the magnitude of transgenerational effects of high temperature on hatching success of eggs and hatchling survival were similar to its direct effects. This indicates that heat waves can affect natural populations through transgenerational effects, and that the magnitude of such effects can be equally strong to the direct effects of temperature, although this depends on the trait considered. Our results highlight the importance of considering transgenerational effects of climate warming when estimating its effects in the wild.