We investigate languages, where the number of words of every length is
bounded from above by a constant independent of the length. The issue
s involved are very basic in the theory of formal languages and, moreo
ver, have arisen in some recent studies in cryptography. This paper de
als with closure properties of such ''slender'' languages with respect
to a number of operations, some of them introduced very recently. Clo
sure is considered both in general and with respect to slender languag
es in the families of the Chomsky hierarchy.