IS STRING THEORY A THEORY OF STRINGS

Recently a great deal of evidence has been found indicating that type IIA string theory compactified on K3 is equivalent to heterotic string theory compactified on T-4. Under the transformation which relates th e two theories, the roles of fundamental and solitonic string solution s are interchanged. We show that there exists a solitonic membrane sol ution of the heterotic string theory which becomes a singular solution of the type lIA theory, and should therefore be interpreted as a fund amental membrane in the latter theory. We speculate upon the implicati ons that the complete type IIA theory is a theory of membranes, as wel l as strings.