This article examines the third language (L3) developing morphology of 78 Basque-Spanish bilinguals following a Content and Language Integrated Learning (CLIL) program and a mainstream English as a foreign language (non-CLIL) program. The analysis of cross-sectional and longitudinal oral data shows that (1) the omission of inflection in the L3 English interlanguage of these groups of learners is due to problems with the realization of surface morphology, (2) there is a dissociation in frequency of suppliance between suppletive inflection (copula and auxiliary "be") and affixal inflection (the third person morpheme "-s" and the past tense morpheme "-ed") already attested in L2 data, and (3) no significant differences were found between the two groups tested as far as the development of suppletive and affixal tense and agreement morphemes. The overall findings seem to support full-UG explanations for the variable use of morphology in the acquisition of non-native systems. (Contains 6 tables and 9 notes.)