Although many studies of speech errors have been presented in the literature, most have focused on errors occurring at either the segmental or feature level. Few, if any, studies have dealt with the prosodic structure of errors. This paper aims to fill this gap by taking up the issue of prosodic structure in Japanese speech errors, with a focus on the foot level. The 501 speech errors from normal spontaneous speech of Japanese speakers will be compared with those collected from five aphasic patients. The acquisition data of Japanese supports the unmarkedness of foot binarity. Two types of evidence have been presented for this: bimoraic minimality effects in monomoraic lexical items, and disyllabic maximality effects of multisyllabic words. An analysis of the speech errors in normal speech also shows a similar tendency. If one focuses on the deletion/insertion data, one finds that foot binarity plays a crucial role in predicting the site where deletion/insertion occurs. It was found that, in most cases, deletion/insertion of morae (syllables) occurs as a repair strategy at the foot level. A preliminary study of the pathological data of the aphasic patients also indicates that foot binarity plays a role in accounting for this type of data. It is concluded that the results obtained from acquisitional and pathological data strongly support the unmarked status of binary feet. (Contains 19 figures and 10 tables.)