Abstract Nuclei isolated from various sources are capable of protein and RNA synthesis ( Allfrey et al. , 1964 ; Birnstiel et al. , 1962 ; Rendi, 1960; Malec et al. , 1964 ; Sirlin and Schor, 1962; Allfrey and Mirsky, 1962; Rho and Chipchase, 1962; Mittermayer et al. , in press ; Rozijn et al. , 1964 ; Rees and Rowland, 1961). Incorporation of precursors into DNA in isolated nuclei has also been reported ( Friedkin and Wood, 1956; Mazia and Hinegardner, 1963; Behki and Schneider, 1963), but little incorporation was observed without added primer. Furthermore, no evidence was obtained to indicate that any of these nuclear preparations were autonomous with regard to control of DNA synthesis. This communication reports the synthesis of DNA, without added primer, by isolated nuclei of Physarum polycephalum , in which synchronous mitosis occurs naturally. Synthesis occurred only in nuclei isolated during the DNA-synthesizing period of the mitotic cycle; this indicates that these nuclei retain, after isolation, the same mechanism for control of DNA synthesis as is exerted in vivo . The implications of these findings for DNA replication are discussed.