The magnetic properties of poly(phenylacetylene)s with radical groups at the paraposition, poly[(3,5-di-tert-butyl-4-oxyphenyl)acetylene] (1),poly[[4-[(3,5-di-tert-butyl-4-oxyphenyl)(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)methyl]phenyl]acetylene] (2), and poly[[4-(N-tert-butyl-N-oxyamino)phenyl]acetylene] (3), were studied with a SQUID magnetometer. The magnetization and magnetic susceptibility indicated unexpected antiferromagnetic interactions between the radical sites. The exchange interaction observed in these conjugated polyradicals was discussed in connection with their computationally modeled structures.