This paper examines the formal means by which Czech distinguishes transitive and intransitive verbs, and specifically the role of the particle "se" in the process usually called "derived intransitivization.""Se" is shown to perform a number of functions which preclude its being called simply an "intransitivizing particle." By way of comparison, a study by Babby is cited which, using Russian data, transformationally introduces the intransitivizing particle "-sja" onto verbs whose object noun phrase has been preposed, deleted, or otherwise vacated. Parallel examples are drawn from Czech illustrating reflexive, passive, and middle voice constructions, and special attention is paid to middle voice forms in Czech which fail to introduce "se." Furthermore, examples of Czech active voice verbs using "se" which take apparent "direct" objects in the accusative case are discussed. The suggestion is made that in some cases there is competition between verbs occurring with the particle "se" and those taking "si" (the enclitic dative reflexive), and specifically that the presence of "se" (like "si") is sometimes felt as merely a modality element, a point of reference, or a means of emphasis. It is concluded that the sources for Czech "se" are more varied than those for Russian "-sja," and the attempt is made to describe some of these other sources. (Author/AM)