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 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.