We carried out photopumping measurements for the dilute thin films of linear-chain-structured laser dyes where pi-units such as benzene, ethylene, and oxazole are linearly linked via sigma-bonds. Among them, 4-4'-bis[4-(di-p-tolylamino)styryl]biphenyl recorded the lowest stimulated-emission threshold of 2 muJ/cm(2). It was revealed that the threshold was related to whether the number of constituent pi-units was even or odd.