1. This study provides a detailed account of the morphology of the mouthparts of larval lobsters (Homarus americanus) and the setae they bear. The results describe the types of setae found on the mouthparts, present a scheme for their classification, describe the distribution of the various setal types, and present observations of interesting or previously overlooked features of the mouthparts themselves. 2. A scheme of classification (based on the external morphology of the setae, particularly the nature and distribution of the setules) has been devised to describe and categorize the types of setae found on the mouthparts. The setal complement may be arranged into ten major categories, in some of which the variation is sufficient to warrant subdivision into several types of setae. 3. Detailed descriptions of the mouthparts and the distribution of their setae are presented and major developmental trends are noted. Changes in the mouthparts which occur as lobsters pass through the first four stages generally include increases in the size of mouthparts and in the number of setae they bear. Of special interest is the transformation of the comparatively delicate teeth of the first-stage mandible into the massive molars of the fourth stage. The coordinated development of teeth on the gnathal lobe of the mandible and on the ischium of the third maxilliped emphasizes the coordinated manner in which these appendages function. It is evident that the development of the mandibles and third maxillipeds enables lobsters to deal successfully with the more substantial food they encounter in the benthic environment, which they usually enter at the fourth or fifth stage. 4. The various functions and possible functions (for example, chemosensory, tactile, and mechanical) of the setae borne on the mouthparts are discussed in light of the available functional and physiological evidence. The possibility of filter feeding in lobsters, particularly in the constantly-swimming, planktonic larval forms, is considered.