Light transmittance and NMR self-diffusion measurements were carried out on a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer, PE6400 (EO(13)PO(30)EO(13)), in gels and solutions of agarose in 1.0 M NaSCN. The influence of agarose on the clouding behavior and the diffusion of PE6400 as well as the effect of PE6400 on the gelation behavior of agarose were studied. In the presence of agarose coils the cloud-point of PE6400 on cooling is decreased due to the incompatibility of the agarose/PE6400 couple. The cloud-point of PE6400 on heating in agarose gels is situated in the vicinity of the cloud-point of PE6400 in the absence of agarose, indicating a much less repulsive interaction between agarose in the gel state and PE6400. On cooling the mixtures, a large decrease in transmittance was observed at a temperature above the gelation temperature of agarose. This temperature was identified as the critical micellization temperature (cmt) of PE6400. The large decrease in transmittance at the cmt was caused by the formation of aggregates due to the strong incompatibility between agarose coils and hydrophobic impurities present in the PE6400 system. Above the cmt these impurities were solubilized in the PE6400 micelles. The observed self-diffusion of PE6400 in agarose/1.0 M NaSCN systems is faster in the gel (more open structure) than in the solution state. The self-dissusion of the block copolymer in the presence of agarose is lowered compared to the agarose-free case due to the obstruction effect. The presence of 1.0 M NaSCN impedes the formation and growth of the PE6400 micelles on increasing temperature compared to salt-free solutions of PE6400 in D2O.