Onomatopoeia are disproportionately high in number in infants' early words compared to adult language. Studies of infant language perception have proposed an iconic advantage for onomatopoeia, which may make them easier for infants to learn. This study analyses infants' early word production to show a phonological motivation for onomatopoeia in early acquisition. Cross-linguistic evidence from 16 infants demonstrates how these forms fit within a phonologically-systematic developing lexicon. We observe a predominance of consonant harmony and open cv syllables in infants' early words--structures that are typical of onomatopoeia across languages. Infants' acquisition of onomatopoeia is shown to be driven by a preference for structures that are easy to plan and produce. These data present an original perspective on onomatopoeia in early development, highlighting the role of production in language acquisition in general, and onomatopoeic words in particular.