Nuclear envelope invaginations occur in many kinds of cell. Double-labeling of 3T3 cells with Hoechst 33342 strain for DNA and phalloidin-rhodamine for F-actin, show that some nuclei appear to contain tangled knots of F-actin. Concanavalin A-fluorescein staining for membranes shows that the knots are continuations of the nuclear envelope. Although they contain F-actin, the knots appear by electron microscopy to be cytoplasmic invaginations lacking microfilaments. Since we have shown previously that nuclear-membrane associated actin forms perinuclear shells in 3T3 cells, we propose that nuclear knots also are composed of actin associated with the nuclear membrane. 3T3 nuclei also contain nuclear invaginations of a second kind. These invaginations lie perpendicular to the first type and lack F-actin.