Isolated galaxies seem to have more nuclear activity and less disk activity than nonisolated objects. This is interpreted as an expression of the mass-profile in their halos.

The discovery of voids in the three-dimensional distribution of galaxies has rapidly been followed by claims that in at least one such void some objects, like Markarian and emission-line galaxies, are present. Why are only active galaxies found in these voids? Should we also expect the opposite to be true, namely that active galaxies ought to be found more often in voids, or in regions of the Universe with lower-than-average density? There are indications that this seems to be the case, as Gisler (1978) found that emission-line galaxies, Markarians and possibly Seyferts avoid environments of dense clusters. Differences between “field” and “cluster” populations are probably due more to the enhanced local density of galaxies than to the nature of the overall environment.