We study the dynamics of active particles advected by 3‐dimensional open incompressible flows. We find that 3D reactive flows have fundamentally novel dynamical features, not found in 2D systems. In particular, we argue that that the reaction’s productivityper reaction stepcan be enhanced, with respect to the 2D case, while the productivityper unit timein some 3D flows goes to zero in the limit of high mixing rates, in contrast to the 2D behavior, in which the productivity goes to a finite constant. © 2004 American Institute of Physics