This study examines the storage versus composition of Spanish inflected verbal forms in first language (L1) and second language (L2) speakers of Spanish. L2 participants were selected to have mid-to-advanced proficiency, high classroom experience, and low immersion experience, typical of medium-to-advanced foreign language learners. Participants were shown the infinitival forms of verbs from either Class I (the default class, which takes new verbs) or Classes II and III (nondefault classes) and were asked to produce either first-person singular present tense or imperfect forms, in separate tasks. In the present tense, the L1 speakers showed inflected-form frequency effects (i.e., higher frequency forms were produced faster, which is taken as a reflection of storage) for stem-changing (irregular) verb forms from both Class I (e.g., "pensar-pienso") and Classes II and III (e.g., "perder-pierdo"), as well as for non-stem-changing (regular) forms in Classes II/III (e.g., "vender-vendo"), in which the regular transformation does not appear to constitute a default. In contrast, Class I regulars (e.g., "pescar-pesco"), whose non-stem-changing transformation constitutes a default (e.g., it is applied to new verbs), showed no frequency effects. L2 speakers showed frequency effects for all four conditions (Classes I and II/III, regulars and irregulars). In the imperfect tense, the L1 speakers showed frequency effects for Class II/III (-"ia"-suffixed) but not Class I (-"aba"-suffixed) forms, even though both involve non-stem-change (regular) default transformations. The L2 speakers showed frequency effects for both types of forms. The pattern of results was not explained by a wide range of potentially confounding experimental and statistical factors and does not appear to be compatible with single-mechanism models, which argue that all linguistic forms are learned and processed in associative memory. The findings are consistent with a dual-system view in which both verb class and regularity influence the storage versus composition of inflected forms. Specifically, the data suggest that in L1, inflected verbal forms are stored (as evidenced by frequency effects) unless they are "both" from Class I "and" undergo non-stem-changing default transformations. In contrast, the findings suggest that at least these L2 participants may store all inflected verb forms. Taken together, the results support dual-system models of L1 and L2 processing in which, at least at mid-to-advanced L2 proficiency and lower levels of immersion experience, the processing of rule-governed forms may depend not on L1 combinatorial processes, but instead on memorized representations. (Contains 4 figures, 2 tables and 19 notes.)