We examined the phonetics and phonology of intonation of infant-directed speech (IDS) and non-IDS in story-reading in two typologically-divergent languages, English and Bengali. In addition to finding an increase in f0 range and variability in IDS, replicating previous work on IDS prosody, we found novel evidence that f0 manipulations in IDS are constrained by intonational phonology. Speakers in both languages used an increased proportion of tonal elements with higher tonal targets and more turning points in IDS, within the language-specific intonational grammar. The tonal elements showing increased use in IDS also were associated with marking topic and focus. Thus, phonetic changes in IDS may in part be induced by speakers’ choices of phonological tonal elements, which in turn may be connected with choices about marking discourse structure.