  We solve the coupled Einstein-Vlasov system in spherical symmetry using
direct numerical integration of the Vlasov equation in phase space. Focusing on
the case of massless particles we study critical phenomena in the model,
finding strong evidence for generic type I behaviour at the black hole
threshold that parallels what has previously been observed in the massive
sector. For differing families of initial data we find distinct critical
solutions, so there is no universality of the critical configuration itself.
However we find indications of at least a weak universality in the lifetime
scaling exponent, which is yet to be understood. Additionally, we clarify the
role that angular momentum plays in the critical behaviour in the massless
case.
