Summary1All immature stages and adults of both sexes ofCimex lectulariusL. will feed on man, mouse, and domestic fowl. The insect can be bred to the adult stage on any of these hosts, and such adults will mate and produce fertile eggs.2The speed of development from egg to adult is slightly greater on mouse than on man or fowl under the conditions of the experiments. Given equal opportunity for feeding once in every instar, a greater proportion of bugs reach maturity on mouse than on man, the lowest proportion with any of the three hosts occurring on fowl.3Facts are brought forward to show that the shorter period for development on mouse is correlated with a relatively heavy blood‐meal. Also, adults bred on mouse are heavier than those bred on fowl or on man. Although bugs fed on fowl develop more quickly than on man, they do not always take heavier meals.4Bugs bred on mouse produce more eggs at the first feed than bugs bred on man or fowl. This increased egg‐production is associated with the greater body‐weight of mouse‐bred bugs: but there appears to be also another factor which makes mouse a better host for the production of bed‐bug eggs.5Mated adults and 2nd‐instar nymphs, when starved on a single blood‐meal, live longer on man than on either mouse or fowl. This is not obvious on the unmated adults until the variation in length of life associated with weight of meal is eliminated: then it is seen that unmated bugs, too, live longest when fed on mouse.6The blood of man, when dried, contains a greater proportion of dry material than the blood of either mouse or fowl, This may account for the longer life of bugs starved on human blood.7The differences in the life of bed‐bugs bred on the fowl and on man are small. Those fed on man, however, appear to be slightly better fitted for survival on one meal from the host under the conditions of the experiments than bugs fed on fowl.8The aerences in the life of bed‐bugs living on different hosts—man, mouse, and fowl—in nature are more likely to be due, primarily, to differences in the behaviour of the insects in the presence of those hosts rather than to thephysical effects of the meals taken from them. It is thought that the slitly different results with insects fed on the two human hosts may be due rather to differences in feeding behaviour of the insect than to the effe