  We observe critical phenomena in spherical collapse of radiation fluid. A
sequence of spacetimes $\cal{S}[\eta]$ is numerically computed, containing
models ($\eta\ll 1$) that adiabatically disperse and models ($\eta\gg 1$) that
form a black hole. Near the critical point ($\eta_c$), evolutions develop a
self-similar region within which collapse is balanced by a strong,
inward-moving rarefaction wave that holds $m(r)/r$ constant as a function of a
self-similar coordinate $\xi$. The self-similar solution is known and we show
near-critical evolutions asymptotically approaching it. A critical exponent
$\beta \simeq 0.36$ is found for supercritical ($\eta>\eta_c$) models.
