In this paper an analysis has been carried out of the known ways of setting the small fishes' backs in the same direction, and the fundamentals of the new method of transversal orientation are presented. An introductory theoretical analysis is given for the transversal orientation method, using a pair of parallel, inclined to the level, plain rollers rotating in the same direction. The new method has been verified by experiments on the Baltic sprats. Due to preliminary nature of the research and lack of the broad theory which could describe the orientation operation by rollers, causality is shown in a form of the multinomial, which approximates the results of the experimental research. The multinomial, irrespective of its statistical coincidence with results of experiments, has been given a logical consideration. The new method has proved to be efficient, and the important factors affecting orientation efficiency were determined. (c) 2007 Elsevier Ltd. All rights reserved.