Abstract: The invention relates to a plastic heat exchanger (1), in particular for automotive applications, having an inlet tank (2) which has an inlet (5) for a first fluid and surrounds an inlet chamber (6), having an outlet tank (3) which has an outlet (7) for the first fluid and surrounds an outlet chamber (8), having multiple plastic tubes (4) which are tightly connected to the inlet tank (2) and to the outlet tank (3) and connect the inlet chamber (6) to the outlet chamber (8) so they communicate with one another and around which a second fluid flows during operation. To improve the heat transfer, at least some of the plastic tubes (4) have at least one longitudinal section (9) in which ring-shaped areas (13) having variable cross sections follow one another in the longitudinal direction of the tube.