The conformational structure and the interchain aggregation behavior of a semirigid conjugated polymer bearing a decyl side chain, poly(2,3-diphenyl-5- decyl-l,4-phenylenevinylene) (DPlO-PPV), in solutions with chloroform and toluene have been investigated by means of small-angle neutron scattering (SANS), static light scattering (SLS) and dynamic light scattering (DLS). The radius of gyration, persistence length, and the second virial coefficient of the polymer in dilute solution as determined by SLS were higher in chloroform than in toluene; consequently, the polymer assumed a more extended wormlike chain conformation in the former. The difference in the strength of interaction in the two solvents gave rise to contrasting aggregation behavior of the polymer in the semidilute regime. While only a minor fraction of the polymer underwent segmental association in chloroform, a considerable fraction of it formed clusters (microgels) with several micrometers in size in toluene. These clusters were further found to consist of sheetlike nanodomains. Compared with the DP-PPV bearing a shorter hexyl side chain, DP6-PPV, the aggregates of DPlO-PPV in toluene were weaker as they could be easily disrupted by moderate heating. This was attributed to a lack of strong π-π interaction between the DPlO-PPV segments due to the greater steric hindrance imposed by the longer decyl side chains.