Rules, and exceptions to such rules, are ubiquitous in many domains, including language. Here we used simple artificial grammars to investigate the influence of 2 factors on the acquisition of rules and their exceptions, namely type frequency (the relative numbers of different exceptions to different regular items) and token frequency (the number of exception tokens relative to the number of regular tokens). We familiarized participants to either a prefixation pattern (where regulars started with /ZaI/ and exceptions ended with /ZaI/) or a suffixation pattern (where regulars ended with /ZaI/ and exceptions started with /ZaI/). We show that the type and the token frequency of regular items and exceptions influence in different ways what participants can learn. For the exceptions to be learned, they have to occur sufficiently often so that participants can memorize them; this can be achieved by a high token frequency. However, a high token frequency of the exceptions also impaired the acquisition of the "regular" pattern. In contrast, the type frequency of the patterns seemed to determine whether the regular pattern could be learned: When the type frequency of the regular items was sufficiently high, participants successfully learned the regular pattern even when the exceptions were played so often that 66% of the familiarization items were exceptions. We discuss these findings in the context of general learning mechanisms and the role they may play in language acquisition. (Contains 2 tables, 1 footnote and 12 figures.)