Transcriptions of 17 hours of recordings of babbling by fraternal twins in an English/Serbian language environment (1,454 utterances) were analyzed for basic aspects of articulatory organization, effects of the “twin situation,” and effects of the two ambient languages, English and Serbian. Predictions that babbling would be dominated by a “frame” provided by rhythmic mandibular oscillation were, for the most part, confirmed in the form of consonant-vowel co-occurrence constraints showing little active intersegmental tongue movement (one subject) and a predominance of “vertical” (mandible-induced) intersyllabic variegation (both subjects). A possible effect of the twin situation was observed in the form of unusually high frequencies of the consonants and vowels most frequent in babbling. The only prominent ambient language effect was a relatively high frequency of palatal glides (palatals are common in Serbian).