The generation time (time for doubling of the population) under optimum conditions for 28 conventional organisms ranging from bacteria to redwood trees increased at a fairly regular rate with increase in body weight. The average generation time was about $$\root 4.5 \of 10^{20.4}X$$ or X$$^{0.227} × 10^{4.4}$$ minutes when X was the body weight in grams. The generation times of two bacteriophages were very close to this value, but the generation times for five tissue-invading viruses were much greater. On this basis and on the basis of plausible corrections for tissue-invading viruses, it is believed that the relation of body weight to rate of reproduction is basically the same for viruses as for conventional organisms.