Research on the spawning and fecundity of the Argentinean mackerel Scomber japonicus marplatensis was carried out during the spawning season 1966/67. The purpose of the present study was to obtain data for the estimation of the number of eggs which can be produced and spawned by one female of a known size, during one season of reproduction: 152 females were studied. All yolked ova of the biggest size group were taken into consideration. On the basis of the relatively short spawning season, the great homogeneity in meristic characteristics, and histological examination, it is assumed that the Argentinean mackerel spawns once a year. The fecundity of the smallest female of 315 mm in total length was calculated to be 101,859 eggs, and the total number of yolked ova 181,000; for the biggest female of 470 mm in length, 522,370 and 853,500, respectively. The relation fecundity/length and fecundity/weight is almost rectilineal. The equations: F=8.17 е0.008L for fecundity/length relationship, and F=0.42 W1.043 for fecundity/weight relationship, were calculated. Prediction curves were established in each case. It would appear that the fecundity of smaller fish is associated more with length, and the fecundity of bigger fish more with weight. Large differences in fecundity of specimens of the same size were observed.