  The effects of fluid elasticity on the swimming behavior of the nematode
\emph{Caenorhabditis elegans} are experimentally investigated by tracking the
nematode's motion and measuring the corresponding velocity fields. We find that
fluid elasticity hinders self-propulsion. Compared to Newtonian solutions,
fluid elasticity leads to 35% slower propulsion speed. Furthermore,
self-propulsion decreases as elastic stresses grow in magnitude in the fluid.
This decrease in self-propulsion in viscoelastic fluids is related to the
stretching of flexible molecules near hyperbolic points in the flow.
