In this paper, we compute the cohomology with compact supports of a Picard
modular surface as a virtual module over the product of the appropriate Galois
group and the appropriate Hecke algebra. We use the method developed by Ihara,
Langlands, and Kottwitz: comparison of the Grothendieck-Lefschetz formula and
the Arthur-Selberg trace formula. Our implementation of this method takes as
its starting point the work of Laumon and Morel.