Experiments were carried out to determine (1) the degree of agreement with which subjects identify paragraph boundaries in unindented prose passages, (2) if a significant proportion of cues to paragraph structure are formal in nature, (3) if the identification of paragraphs in different kinds of prose differentially depends on semantic (as distinct from formal) cues, and (4) to study developmental changes in paragraphing ability. Nouns, verbs, adjectives and adverbs in ten English prose passages were replaced by nonsense paralogs. The subjects identified paragraph boundaries in both English and nonsense versions. The median reliability for paragraphing English passages was .86; for nonsense, .75. The median correlation between paragraphing of English and nonsense versions of the same passage was .71. In the light of inter-judge reliabilities, it seems clear that the paragraph is a psychological unit. It also seems possible to infer for each passage the relative saliency of the classes of semantic and formal cues, the nature of their interaction, and the univocality of each class. It is suggested that a paragraph is a unit composed of many "structural constituents," each constituent consisting of a string dominated by a common marker--sentence subject (lexical system), verb tense (grammatical system), or generalized statement of a topic (rhetorical system). See ED 016 961 and ED 016 976. (Author/AMM)