The specific growth rate, *G*, is widely used in articles dealing with the growth of aquatic organisms under experimental conditions. When individuals are untagged, the arithmetic mean of *G* for a group of animals must be calculated from weight geometric means, not from arithmetic means. The type of weight mean used in articles to calculate the arithmetic mean of *G* is usually not reported, and an extended use of weight arithmetic means is common. The arithmetic mean of *G* so calculated is biased according to the increment in the squared coefficient of variation of body weights. Another potential bias in the calculation of the arithmetic mean of *G* is size-dependent mortality; this bias cannot be avoided when individuals are untagged, but maximal and minimal values can be obtained. In summary, in view of these analytical results, it seems prudent to calculate the arithmetic mean of *G* for a group of untagged animals from geometric means of weights, and to estimate the maximal error due to the possibility of size-dependent mortality whenever possible. |