  We explore the implications of type Ia supernovae (SNIa) observations on flat
cosmological models whose matter content is an exotic fluid with equation of
state, p=-M^(4(a+1))/rho^a. In this scenario, a single fluid component may
drive the Universe from a nonrelativistic matter dominated phase to an
accelerated expansion phase behaving, first, like dark matter and in a more
recent epoch as dark energy. We show that these models are consistent with
current SNIa data for a rather broad range of parameters. However, future SNIa
experiments will place stringent constraints on these models, and could safely
rule out the special case of a Chaplygin gas (a=1) if the Universe is dominated
by a true cosmological constant.
