The morphological corollaries of flightlessness of the kakapo (Strigops habroptilus) have been studied using skin specimens, skeletons, and pectoral dissection of an anatomical specimen. These have been compared with the closely related, flighted kea (Nestor notabilis), and secondarily with other Psittaciformes and the convergent hoatzin (Cuculiformes: Opisthocomus hoazin). S. habroptilus is the most massive and sexually dimorphic psittaciform in the world, and has the smallest relative wing size of any parrot. Alar pterylography of S. habroptilus is similar to that of other parrots, but remiges of the species are shorter, comparatively rounded, show less asymmetry of vanes, and have fewer interlocking barbules distally. S. habroptilus shows peculiarities of the sternum (vestigial carina, shortened spina externa), coracoid (elongate processus lateralis, enlarged angle with scapula), and humerus (prominent tuberculum ventrale, undercut crista bicipitalis). Pectoral skeletal dimensions of S. habroptilus are smaller than those of N. notabilis, whereas the reverse is true for pelvic dimensions. Most skeletal dimensions of S. habroptilus are more variable (within sexes) than those of N. notabilis. Proximal wing elements are disproportionately long and distal elements disproportionately short in S. habroptilus. The legs of S. habroptilus are characterized by disproportionately long femora and disproportionately short tarsometatarsi. Distinctive features of the pectoral musculature of S. habroptilus include a greatly reduced Mm. pectoralis thoracica and supracoracoideus, the absence of a distinct proximal muscle belly of M. propatagialis tendo longus, an extensive M. cucullaris capitis clavicularis associated with a voluminous crop, and an essentially tendinous M. sternocoracoideus. Relative to mean body mass, all dimensions of the antebrachial, carpometacarpal, digital, and patagial muscles are smaller in S. habroptilus than in N. notabilis. These aspects are compared to those of other flightless birds. Discussed are implications of flightlessness and associated large body size of S. habroptilus for issues of thermodynamics, metabolism, activity patterns, digestive anatomy, diet, reproduction, and insularit. © 1992 Wiley-Liss, Inc.