This single-subject case study evaluates effects of treatment of a complex onset on the sound system of a monolingual Spanish-speaking child (female, aged 3;9) with phonological delay. Pretreatment, the child excluded all consonant+liquid clusters, as well as [the alveolar tap] and trill /r/. Immediately following training on [using the alveolar tap] in non-words, the child generalized across consonant+liquid clusters and the tap singleton. These improvements continued to 2 months post-treatment follow-up, with the ultimate addition of the trill at that point in time. Consonant+glide sequences, whose structural status as complex onsets is debated in the Spanish phonology literature, patterned differently from consonant+liquid sequences. Specific findings are viewed in light of linguistic markedness, syllable structure, sonority sequencing, and the representation of consonant clusters.