Abstract. Existing black box and other algorithms for explicitly recognising groups of Lie type over GF(q) have asymptotic running times which are polynomial in q, whereas the input size involves only log q. This has represented a serious obstruction to the efficient recognition of such groups.Recently, Brooksbank and Kantor devised new explicit recognition algorithms for classical groups; these run in time that is polynomial in the size of the input, given an oracle that recognises PSL(2, q) explicitly.The present paper, in conjunction with an earlier paper by the first two authors, provides such an oracle. The earlier paper produced an algorithm for explicitly recognising SL(2, q) in its natural representation in polynomial time, given a discrete logarithm oracle for GF(q). The algorithm presented here takes as input a generating set for a subgroup G of GL(d, F ) that is isomorphic modulo scalars to PSL(2, q), where F is a finite field of the same characteristic as GF(q); it returns the natural representation of G modulo scalars. Since a faithful projective representation of PSL(2, q) in cross characteristic, or a faithful permutation representation of this group, is necessarily of size that is polynomial in q rather than in log q, elementary algorithms will recognise PSL(2, q) explicitly in polynomial time in these cases. Given a discrete logarithm oracle for GF(q), our algorithm thus provides the required polynomial time oracle for recognising PSL(2, q) explicitly in the remaining case, namely for representations in the natural characteristic.This leads to a partial solution of a question posed by Babai and Shalev: if