Self-consistent simulations of seven groups of galaxies with halos have been performed to find a constraint upon the size of missing halos around spiral galaxies. An initial galaxy, which consists of 100 superstars, has half-mass radius 41 kpc and central velocity dispersion 235 km s−1. The simulations start from the epoch of maximum expansion. The initial conditions involve a variety of spatial distributions of galaxies, and the velocity dispersion of galaxies as would be permitted for maximum expansion. Dense groups having collapse times shorter than (2/3)H 0 −1 are shown to form multiple mergers in a Hubble time H 0 −1 . From a comparison of the frequencies of cD galaxies, or multiple mergers, in observed and simulated groups, it is concluded that the effective radius of missing halos is less than 41 kpc.