Background: Five-year-olds with specific language impairment (SLI) often struggle with mastering grammatical morphemes. It has been proposed that verbal morphology is particularly problematic in this respect. Previous research has also shown that in young typically developing children grammatical markers appear later in more phonologically challenging contexts. Aims: The main aim was to explore whether grammatical deficits in children with SLI are "morphosyntactic" in nature, or whether "phonological" factors also explain some of the variability in morpheme production. The analysis considered the effects of the same phonological factors on the production of three different morphemes: two verbal (past tense "-ed"; third-person singular "-s") and one nominal morpheme (possessive "-s"). Methods & Procedures: The participants were 30 children with SLI (21 boys) aged 4;6-5;11 years (mean = 5;1). The data were collected during grammar test sessions, which consisted of question/answer elicitations of target forms involving picture props. A total of 2301 items were analysed using binary logistic regression; the predictors included: (1) "utterance position" of the target word, (2) "phonological complexity" of its coda, (3) "voicing" of the final stem consonant, (4) "syllabicity" (allomorph type) and (5) "participant" accounting for the individual differences in the responses. Outcomes & Results: The results showed a robust effect of "syllabicity" on the correct morpheme production. Specifically, syllabic allomorphs (e.g., She "dresses") were significantly more challenging than the segmental ones (e.g., He "runs") for all three morphemes. The effects of other factors were observed only for a single morpheme: coda "complexity" and "voicing" helped explain variability in past tense production, and "utterance position" significantly affected children's performance with the possessive. The "participant" factor also had a significant effect, indicating high within-group variability--often observed in SLI population. Conclusions & Implications: The systematic effect of "syllabicity" across both verbal and nominal morphemes suggests morphophonological influences in the grammatical development of children with SLI that cannot be fully explained by syntactic deficits. Poorer performance in producing syllabic allomorphs can be accounted for by much lower overall frequency of these forms, and by the "tongue-twisting" effect of producing similar segments in succession, as in "added" [aed?d], "washes" [w???z]. Interestingly, the greater acoustic salience of the syllabic allomorphs (an extra syllable) does not enhance children's abilities to produce them. These findings suggest that the interconnections between different levels of language have a stronger effect on the grammatical development of children with SLI than might be expected. "Allomorphy" should, therefore, be taken into account when designing language assessments and speech therapy, ensuring that children receive sufficient practice with the entire set of allomorphic variants.