  We give a negative answer to Ulam's Problem 19 from the Scottish Book asking
{\it is a solid of uniform density which will float in water in every position
a sphere?} Assuming that the density of water is $1$, we show that there exists
a strictly convex body of revolution $K\subset {\mathbb R^3}$ of uniform
density $\frac{1}{2}$, which is not a Euclidean ball, yet floats in equilibrium
in every orientation. We prove an analogous result in all dimensions $d\ge 3$.
