A compilation of values for the exponential coefficient of natural mortality (M) is given for 175 different stocks of fish distributed in 84 species, both freshwater and marine, and ranging from polar to tropical waters. Values of L∞(LT, cm), W∞(g, fresh weight), K (1/year) and T (°C, mean annual water temperature) were attributed to each value of M, and the 175 sets of values plotted such that: 1) log M = −0·2107 − 0·0824 log W∞ + 0·6757 log K + 0·4627 log T and 2) log M = −0·0066 − 0·279 log L∞ + 0·6543 log K + 0·4634 log T The multiple correlation coefficients are for 1) 0·845, and for 2) 0·847, while the critical value (171 d.f.) is 0·275 (for P = 0·01). All slopes are significantly ≠ 0 (for P = 0·001). The standard deviation of estimates of log M are for 1) 0·247, and for 2) 0·245. The equations provide highly reliable estimates of M for any given fish stock, given the values of W∞ or L∞ and K of the von Bertalanffy growth formula, and an estimate of the mean water temperature in which the stock in question lives. Only two groups have values of M generally differing from those obtained through the proposed equations: the Clupeidae, with generally lower and the polar fishes with generally higher values. Correction factors are given for both groups. Potential applications of the findings to population dynamics are discussed together with some ecological implications.