In mouse astroglia colony cultures, cells display dramatic changes in their 'social behavior' during differentiation. The non-motile glioblasts progress through highly motile pro-astroblast and astroblast stages before becoming non-motile fibrous astrocytes. These changes in the behavior of astroglia are paralleled by changes in the organization of their microfilaments. Immunofluorescence staining of the astroglia colony cultures with rhodamine phalloidin, which binds to actin filaments (F-actin) formed by any of the 6 actin isoforms and with monoclonal antibody to alpha-smooth muscle (alpha-sm) actin (an actin isoform typical of smooth muscle cells), shows that the majority of the cells are positive for both alpha-sm actin and F-actin. Approximately 15% of the cells, however, were negative for alpha-sm actin but stained positively for F-actin, indicating that the actin of these cells belongs to isoforms other than the alpha-sm isoform.