5 studies examined contributions of syllable-ordering and rhythmic properties of syllable strings to 6- and 9-month-old infants' speech segmentation. A pair of methods measuring complementary properties of representational units was used: a noise detection task sensitive to perceived cohesiveness of pairs of syllables, and a discrimination maintenance task sensitive to compactness of representations of syllable pairs. For 9-month-olds, results show that a key pair of syllables was represented as a unit when the grouping of these syllables was supported by correlated regularities of ordering and rhythm in the set of stimulus strings, but not when such grouping was supported by only rhythmic or only syllable-ordering regularity. For 6-month-olds, results show that a key pair of syllables was represented as a unit whenever grouping was supported by rhythmic regularity in the stimulus strings, regardless of whether syllable-ordering regularity was also present. Thus, whereas 9-month-olds appear to be capable of integrating sequential and suprasegmental information in forming wordlike (multisyllabic) phonological percepts, 6-month-olds are not. The emergence of integrative abilities portends increased efficiency in speech processing and may contribute to the formation and use of an initial lexicon.