The acellular slime mold Physarum polycephalum has been exposed continuously to a variety of low-level, extremely-low-frequency (ELF) fields for periods ranging from two months to five years. Changes in several biological parameters have been observed that are significant (P < 0.05) and reproducible. Cultures were exposed to 75-, 60-, and 45-Hz CW, and 76-Hz frequency-modulated fields. Electric-field intensities ranged from 0.04 to 0.7 V m−1 (rms); magnetic fields, from 0.01 to 0.2 milliteslas (rms). The observed changes are generally those of a slowing of cellular processes. A longer nuclear-division cycle and depressed respiration rate have been seen under exposure to most CW fields and to all frequency-modulated fields that have been tested. Additional experiments indicate that lengths of the nuclear-division cycle of cultures formed by mixing exposed and control samples lie between those of control and exposed cultures. Indirect measurements of chromosomal numbers of this polyploid organism indicate no statistically significant difference between exposed and control nuclei.