Modulators of sphingosine phosphate receptor-1 (S1P₁) have recently been focused as a suppressant of autoimmunity. We have discovered a 4-ethylthiophene-based S1P₁ agonist 1-({4-Ethyl-5-[5-(4-phenoxyphenyl)-1,2,4-oxadiazol-3-yl]-2-thienyl}methyl)azetidine-3-carboxylic acid (CS-2100, 8) showing potent S1P₁ agonist activity against S1P₃ and an excellent in vivo potency. We report herein the synthesis of CS-2100 (8) and pharmacological effects such as S1P₁ and S1P₃ agonist activity in vitro, peripheral blood lymphocyte lowering effects and the suppressive effects on adjuvant-induced arthritis and experimental autoimmune encephalomyelitis (EAE) in animal models. The pharmacokinetic data were also reported. CS-2100 (8) had >5000-fold greater agonist activity for human S1P₁ (EC₅₀; 4.0 nM) relative to S1P₃ (EC₅₀; >20000 nM). Following administration of single oral doses of 0.1 and 1 mg/kg of CS-2100 (8) in rats, lymphocyte counts decreased significantly, with a nadir at 8 and/or 12 h post-dose and recovery to vehicle control levels by 24–48 h post-dose. CS-2100 (8) is efficacious in the adjuvant-induced arthritis model in rats (ID₅₀; 0.44 mg/kg). In the EAE model compared to the vehicle-treated group, significant decreases in the cumulative EAE scores were observed for 0.3 and 1 mg/kg CS-2100 (8) groups in mice. While CS-2100 (8) showed potent efficacy in various animal disease models, it was also revealed that the central 1,2,4-oxadiazole ring of CS-2100 (8) was decomposed by enterobacteria in intestine of rats and monkeys, implicating the latent concern about an external susceptibility in its metabolic process in the upcoming clinical studies.