The influence of shear rate on the primary nucleation of para-amino benzoic acid in water has been investigated via a series of cooling crystallization experiments. For each experiment, we recorded the induction time at various temperatures and supersaturation ratios, employing two flow devices: a capillary tube in which the solution was divided into hundreds of monodisperse droplets and a set of stirred vials. The capillary tube was used to perform experiments in stagnant conditions (motionless droplets) and low shear rate conditions (flowing droplets), while the stirred vials were used to perform experiments at relatively high shear rates. In this way, a wide range of shear rates was investigated. Comparing the results obtained for the motionless and flowing droplets, we saw that the nucleation rate is significantly increased (by several orders of magnitude) by the shear field; however, when the shear rate increases beyond a certain level (stirred vials experiments), we observed a drop in the nucleation rate. Thus, the results demonstrate a non-monotonic dependence of primary nucleation rate on shear rate. Various mechanisms to explain the effect of shear on nucleation are quantitatively and qualitatively discussed; however, at present no definitive conclusion can be drawn to identify the controlling mechanism.