In the period from 1990 to 1992, the landings of horse mackerel (Trachurus symmetricus murphyii) off Central Chile showed a conspicuous seasonality, with a maximum between April and July, and a minimum between September and January. The monthly landings (Y, tons) were fitted to a trigonometrical function, with amplitude (A) being variable and period (T) constant along a long-term trend (a), whose form is: Y_F = a - ATcos(n P t + F). The predicted 1993 monthly landings were calculated, with an accumulated value of 2.5 million tons. The difference between the observed and predicted monthly landings was 0.8%.