The article deals with the distribution of monosyllabic and two-syllabic endings of Instrumental singular case in the prose works of three authors who are significant for characterizing the norm of the literary language of the late 18th – early 19th centuries: N.M. Karamzin, A.S. Shishkov, I.A. Krylov. It is found that, despite the genre differences of the studied texts and the fundamentally different theoretical attitudes of their authors towards the norms of literary language, in the use of two-syllabic vs monosyllabic endings in the Instrumental case their norm is basically the same: two-syllabic endings significantly predominate in all positions, with a sporadic penetration of monosyllabic (colloquial) variants without their special distribution. The monosyllabic ending of 3rd declension nouns -ью is encountered somewhat more often than the ending -ой(-ей), and in some positions in Shishkov’s prose such forms are even more common than in Karamzin’s. The special distribution of monosyllabic and two-syllabic endings of the Instrumental singular case will subsequently be received only in Pushkin’s fiction.