Recent research on the acoustic properties of S-final words in English has revealed unexpected effects of morphology on phonetic realization, especially on acoustic duration (Zimmermann 2016, Plag et al. 2017, Seyfarth et al. 2017, Tomaschek et al. 2019). In this paper, we investigate 462 plural tokens and 417 genitive-plural tokens elicited in a production experiment. The statistical analyses show that plural S is significantly shorter than genitive-plural S. The duration effect is, however, not restricted to the final S, but extends over the whole word. We find that word-form frequency is predictive of duration, resulting in shorter durations for (the more frequent) plurals, and longer durations for (the less frequent) genitive plurals.The empirical findings also have implications for morphological theory. Word-form frequency effects for regularly inflected words in speech production are at odds with theories in which only morphologically irregular words, or highly frequent regular words, are assumed to be stored (e.g. Pinker 1999, Alegre & Gordon 1999). The word-form frequency effect can be more naturally accounted for in word-and-paradigm models of morphology (e.g. Matthews 1974, Blevins 2016), in which individual word-forms may have representations in a network of morphologically related forms.