1. In the estuary of River Tatara, Fukuoka, the mono-specific populations of a burrowing fiddler crab, Ilyoplax pusillus DE HAAN, are usually found on the river-bed of muddy or sandy sediments. By the quadrat census, it was made clear that the distribution of the nests of the crab becomes more and more uniform with the increase of the population density. This fact indicates that the interaction among individuals is promoted at highdensity. 2. The interference between two individuals was investigated experimentally. At first, one individual was introduced into a wooden box(10×10×10 cm). After the first individual had burrowed its nest, the second one was introduced into the same box. Then, the distances between the nests of them and the corner of the box nearest to the first individual were measured. The index of the degree of interference(I)between the two individuals was calculated as follows : [numerical formula] Four sex combinations were used in the experiment ; the first individual was a male and the second one was also a male(♂_1-♂_2), ♀_1(the first individual was a female)-♂_2 (the second one a male), ♂_1-♀_2 (the second a female), and ♀_1-♀_2. The curves of the value "I" plotted against the distance from a corner show that the male has a repulsive character, while the female has attractive and a weak repulsive character. 3. The distribution of the nests in the natural habitat was plotted on a map, and the distance between each nest and its nearest neighbour was measured. The degree of interaction among individuals was measured by comparing the frequency of the shortest distance mentioned above with that of the random one calculated by MORISITA'S method as follows : I=Calculated frequency/Observed frequency. This value "I" was calculated at various nest densities. The curves of value"I"show that the type of interference reveals the repulsive relation among individuals. However, the shape of the curve is constant irrespective of the increase of population density. 4. By comparing the results mentioned above with the interference types observed in the experimental boxes, it is concluded that the appearance of the repulsive type in the distribution of nests in the natural habitat depends mostly on the active character of the males.
