The biogenetic origins of amphidinin A (1) and amphidinolide P (2) were investigated by feeding experiments with C-13-labeled acetates. C-13-NMR data of C-13-enriched samples revealed that the all carbons of 1 and 2 were derived from acetates. The polyketide chain of 1 was formed from one triketide chain, two diketide chains, and three unusual isolated C-1 units derived from C-2 of cleaved acetates, while the polyketide chain of 2 was formed from one pentaketide chain, two acetate units, and three unusual isolated C-1 units derived from C-2 of cleaved acetates. The all branched C-1 units of 1 and 2 were derived from C-2 of cleaved acetates.