Failure to supply inflection is common in adult L2 learners of English and agrammatic aphasics (AAs), who are known to resort to bare verb forms. Among attempts to explain the absence of inflection are competing morphological and phonological explanations. In the L2 acquisition literature, omission of inflection is explained in terms of: mapping (Epstein et al., 1996; Prevost & White, 2000), failed modular interaction (Lardiere, 1998), L1 morpho-syntactic constraints governing the activation of the L2 features (Hawkins & Liszka, 2003), L1 prosodic organization which differs from that of L2 (Goad, et al., 2003), and L1 phonological constraints on final consonant clusters (Lardiere, 2003). In agrammatism, inflectional omission has been linked to: processing (Thompson et al., 2002), accessing (Kehayia et al., 1990), impaired rule implementation (Lee & Thompson, 2005), productive (Bird et al., 2003; Mathews & Obler, 1997) and receptive phonology (de Mornay Davies, 2006). Most, if not all, investigations of inflectional omission have focused on populations whose L1 lacks the syntactic representation of inflection. In this study, we concentrate instead on two groups (L1 Polish learners of L2 English and AAs of L1 English) who can be assumed to have an underlying representation of inflectional material because the L1 already has it; yet they show difficulties in the use of inflection. With regard to production, we therefore asked: What contributes more to the problems encountered by these speakers, morphology or phonology? To test this, we administered an elicited production task varying either the morphological or the phonological complexity of the environment of the inflectional morpheme. We hypothesized that if non-target production of inflection is constrained by morphological factors, we would likely see the following: (1) Both groups would perform better on mono- than bi-morphemic homophones. (2) The participants would differentiate between homophonous morphemes, e.g., PLUR, AGR, POSS and show different degrees of omission for these. (3) There would be no significant difference between inflection of mono-syllabic existing verbs and mono-syllabic pseudo-verbs because the rule attaching inflection is present and intact. On the phonological side, we made the following predictions: (1) Sonority of the final segment of stems would affect the production of inflection. (2) Syllabic suffixes would be produced more accurately than their non-syllabic counterparts because of their saliency. (3) Shorter (mono-syllabic) verbs would be affixed better than longer (bi- and tri-syllabic) ones. Results show that in the production of inflection, similar patterns were found in these two fundamentally different populations, L2 learners and AAs. Morphological constraints seem to play a greater role in the omission of inflection than phonological ones. On the other hand, phonology is used by both groups as a compensatory strategy to preserve inflection. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]