An attempt was made to estimate the catching efficiency of a bottom drift net, gensikiami, for kuruma prawn, Penaeus japonicus, from the catch data in the fishing experiments and the underwater observation, which were carried out in Mikawa Bay. The experimental fishing operations were repeated several times for a short duration of time in the limited area of the fishing ground. The catch number of the prawn per unit swept area decreased gradually with each subsequent operation. An analysis of the catch data gave 0.40 to 0.72 as the estimates of the efficiency of the gear for the prawn. The relation between the efficiencies and the body length compositions obtained in each experiment seemed to show that the gear had a mesh selectivity. The standing crops of the prawn estimated by the underwater observation and the experiment showed agreement with each other in order.