An electron spin resonance study of the intermediate radicals formed in the cyclopolymerization of certain nitrogen-containing monomers has been carried out. An initiation radical, both in N-(p-bromophenyl)dimethacrylamide (BrPhDMA) and sym-dimethacryloyldimethylhydrazine (DMMH), could be observed. The crystalline structure of DMMH is not known at present, but the comparison of the crystalline structure of BrPhDMA with that of N-methyldimethacrylamide suggests that the crystalline structure in these cyclization reactions is very important, In the glassy state of BrPhDMA, an anion radical of the methacryl group was detected as was the case in the glassy state of propyl- and benzyldimethacrylamides. It is significant that only the 5-membered cyclic radical was detected in the glassy state of BrPhDMA, because only the non-cyclic initiation radical was observed in irradiated crystalline BrPhDMA at -196°C, and only the 5-membered cyclic radical was detected in propyl- and benzyldimethacrylamide in both the crystalline and glassy states.