Some observations on the biology ofMastacembelus armatus(Lacepède) from Muzaffarnagar (U.P., India) have been made during the period July 1966 to June 1969. At a particular stage of maturity of the fish there is a close relationship between the gonad weight and the body weight which helps in determining the breeding period of the fish. Both male and female fishes mature simultaneously. The gonad weight influences the relative condition, while the visceral weight does not exert any significant effect on it. The development of the female gonads greatly affect the curve of relative condition, more than the development of male gonads, though the curve for male fish also shows a steady rise and fall in the seasonal variations of the relative condition. Seasonal fluctuations in relative condition correspond to its spawning season. InM. armatus, the presence of two peaks in the curve confirm two breeding seasons. The present studies have also elucidated to a certain extent the important aspect of the maturation and spawning habits ofM. armatusin North India. The fish has two breeding seasons of short duration during June and July and again in November. The mature ovaries of each season contain slight maturing ova of next season along with mature ova of the present season. The entire stock of ova measuring 0.16 mm and above are shed during spawning