The effect of predatory water spiders on the tadpole size-structure is poorly understood. Here I present a study of the effect of the spider Thaumasia sp. on the survivorship of Physalaemus pustulosus tadpoles in a tropical system. Under experimental conditions, tadpoles of one or two sizes were offered to juvenile spiders. Two experiments were conducted, one during the day and another during the night. The spiders consumed a greater number of large tadpoles in all treatments, and more tadpoles of both sizes were eaten during the day. This suggests that larger tadpoles are more susceptible to predation by Thaumasia sp. and that tadpoles are more vulnerable during the day.