Summary Cells of different Amoeba proteus strains (C, B and L) lose their ability to attach to the substratum 5, 6 and 7 d, resp., after enucleation. During the first day the attachment rates of enucleated amoebae and anucleate cell fragments are lower than those of whole intact amoebae and nucleate fragments. However, 2 d after the operation, the former ones do not differ in this respect from the latter ones. The comparison of amoebae treated with actinomycin D (1 mg/ml) or puromycin (100 μg/ml) and of enucleated cells shows that protein synthesis plays an important role in maintaining the ability of these protozoans to attach to the substratum. Between 30 min and 4 h after enucleation, the attachment rates of whole enucleated amoebae B and C exceed those of the respective anucleate fragments due to a different behaviour of “anterior” (pre-nuclear) and “posterior” (post-nuclear) anucleate fragments, the attachment rate of the former being significantly higher. However, the “anterior” and “posterior” fragments of L amoebae 2 h after enucleation attach to the substratum with similar rates. 1 d after the operation, “anterior” and “posterior” fragments of amoebae B do not differ in their attachment rates from each other and from whole enucleated amoebae; in strain C, the attachment rate of “posterior” anucleate fragments still remains somewhat lower than that of “anterior” ones. It may be suggested that the role of the cell nucleus in the attachment of amoebae to the substratum comes, first, to supplying the cytoplasm with RNA molecules on which proteins involved in the attachment process are synthesized, and, second, to maintaining the antero-posterior gradient of adhesiveness in moving intact amoebae.