This paper investigates whether and how speakers track the relative frequency of different patterns of alternation in the lexicon, by investigating speakers’ behavior when they are faced with unpredictability in allomorph selection. We conducted a wug test on Seoul Korean verb paradigms, testing whether speakers can generalize reliable lexical patterns. The test was performed in two directions. In forward formation test, the pre-vocalic base and pre-consonantal non-base forms were the stimulus and response, respectively, whereas in backward formation test, the stimulus–response relation was switched. The results show patterns approximating statistical patterns in Seoul Korean verb lexicon, thus confirming the lexical frequency matching reported in many previous studies. However, contrary to the conventional assumption, the results of the backward formation test are consistent with lexical frequencies relevant for the forward formation, not backward formation. This observed asymmetry is broadly consistent with the single base hypothesis (Albright 2002a, b, 2005, 2008), in which forward, as opposed to backward formation rules play a privileged role in speakers’ morphological grammar.