The present study investigated speech production planning from an individual differences perspective. In particular, we explored the possibility that cross-speaker variation in prosodic phrase length—assumed to reflect, in part, variation in speakers’ planning scope—is systematically related to individual differences in working memory capacity—a cognitive resource that early phonological planning may utilize. Connected speech from a read passage, produced by 100 American English speakers, was analyzed for phrase structure, defined within the Autosegmental-Metrical framework, and the lengths of speakers’ intermediate phrases and Intonational Phrases were calculated. Results showed that shorter reading spans (a measure of verbal working memory) were associated with shorter spoken phrase lengths, significantly so in the case of intermediate phrases. The basic findings lend support to the idea that planning is to some extent flexibly dependent on internal and external pressures facing the speaker. We discuss the implications of these findings for models of speech production and models of prosodic interfaces.