  We describe results of a numerical calculation of circularly symmetric scalar
field collapse in three spacetime dimensions with negative cosmological
constant. The procedure uses a double null formulation of the Einstein-scalar
equations. We see evidence of black hole formation on first implosion of a
scalar pulse if the initial pulse amplitude $A$ is greater than a critical
value $A_*$. Sufficiently near criticality the apparent horizon radius $r_{AH}$
grows with pulse amplitude according to the formula $r_{AH} \sim
(A-A_*)^{0.81}$.
