We examine the influence of collective excitations on the transport properties (resistivity and magneto-optical conductivity) for semimetals, focusing on the case of bismuth. We show, using a random-phase approximation (RPA), that the properties of the system are drastically affected by the presence of an acoustic-plasmon mode, which is a consequence of the presence of two types of carriers (electrons and holes) in this system. We find a crossover temperature T∗ separating two different regimes of transport. At high temperatures where T>T∗, we show that Baber scattering explains quantitatively the dc resistivity experiments, while at low temperatures where T