A mutant of Hydra attenuata is analysed, the aberrant, which is distinct from the wild type in having a smaller head with fewer tentacles and only half the number of head-specific cells. The rate of head and foot regeneration and the doubling time are slower in aberrants than in normal hydra. The lower head-forming potential is paralleled by a reduced concentration of head-specific morphogens: compared to the wild type, in the aberrant the concentration of head activator is reduced to 70% in the head and to 50% in the body, the concentration of head inhibitor is reduced to 50% in the head and to 80% in the body. The aberrant is more sensitive (3 times) to added head activator and less sensitive (>5 times) to added head inhibitor than the wild type. The slower rate of foot regeneration is paralleled by a lower content of foot-specific morphogens: compared to the wild type, in the aberrant the foot activator is reduced to 40% and the foot inhibitor to 70%.