The DNA coding for ribosomal RNA in Physarum polycephalum exists as a collection of extra-chromosomal molecules of molecular weight 37 X 10(6). We have investigated the replication of rDNA, with the following results. (a) Replication of rDNA is unscheduled. This means that molecules that are replicated at any particular time in one mitotic cycle have an equal probability of replicating again in each time interval in the subsequent cycle. Similarly, in a single cycle, some molecules replicate more than once, and some not at all. (b) Replication forks appear to move bidirectionally from points 45% or 33% from one end of the DNA. Replicating molecules observed by electron microscopy are all linear.