Let A be a PI-algebra over a field F. We study the asymptotic behavior of t
he sequence of codimensions c(n)(A) of A. We show that if A is finitely gen
erated over F then Inv (A)=lim(n-->infinity) n root c(n)(A) always exists a
nd is an integer. We also obtain the following characterization of simple a
lgebras: A is finite dimensional central simple over F if and only if Inv(A
) = dim A. (C) 1998 Academic Press.