The article characterizes the three types of actants: semantic, deepsyntactic, and surface-syntactic. The discussion, which is carried out in the framework of the meaning-text approach, is based on 1) the distinction between actants and actant slots, as well as 2) an in-depth lexicographic treatment of lexical units involved. Part I of the article deals with semantic actants. The concept of linguistic situation SIT(L), specified by the lexical unit L, and that of obligatory participants of a SIT(L) are introduced. A semantic actant slot is defined by a semantic property (the correspondence to a participant of the SIT(L)) and a lexicosyntactic property (expressibility of this slot in the text). Optional semantic slots and phraseologically-bound circumstantials are discussed; five linguistic criteria are introduced for the choice between a treatment of ‘‘suspect’’ dependents via optional semantic slots or via nonstandard lexical functions (i.e. as phraseologically-bound circumstantials). The article considers obligatory/ optional saturation of semantic actant slots, their blocking, and the diathesis of a lexical unit L — that is, the correspondence between L’s semantic actant slots and deep-syntactic actant slots. Finally, the morphological processes used for increasing/decreasing the number of semantic actant slots of a lexical unit are examined.