In contrast to what is often assumed in universalist approaches to language acquisition, this paper demonstrates that the acquisition of aspect in at least one language, namely in Russian, is not based on an innate primitive category. The results of a comprehension experiment run with Russian preschool children (age 2-6 years) show that Russian aspect is not acquired in a single step, but rather develops as a slow process. This development is further shown to depend on differences in the intrinsic temporal semantics ('Aktionsart') of the verb. Morphological properties of the verb, by contrast, do not seem to play a significant role in the acquisition process. Thus, aspect acquisition relies on an increasing competence in lexical differentiation of Aktionsarten rather than on a direct linking of conceptual primitives with * stimulating discussions, comments and support at various stages of this project. I would also like to thank Johanna Nichols, Elena Lieven and two anonymous reviewers for helpful comments. Daniel Diermeier gave me useful advice for the statistics. A prelimmary version of this paper was presented in July 1996 at the VIIth International Congress for the Study of Child Language in Istanbul, and I thank the audience for the discussion. It goes without saying that I am alone responsible for any remaining mistakes. Harald Schr6pfer and Damel Diermeier helped me with the filming of the experiment and Gerd Klaasen taught me the split screen technique. I am highly indebted to all of them. Furthermore, I am indebted to Natasa Guseva who assisted me in the experiments. I also would like to thank the kindergartens Nr. 13 and 21 in St Petersburg for their extremely valuable support. Galina R. Dobrova provided essential logistic help in St Petersburg without which the research would not have been possible. Downloaded from 352 morphological markers. A very early and prominent role is played by verbs with telic Aktionsart. In line with what has been shown for the acquisition of tense in several languages, the telic Aktionsart might have a bootstrapping effect in the acquisition of aspect, too.