Novel acid addition salts of 6-{2-[1-(6-methyl-3-pyridazinyl)-4-piperidinyl]ethoxy}-3-ethoxy-1,2-benzisoxazole of following formula are disclosed, particularly a crystalline form of a bis-dihydrogenphosphate salt: Such compounds have particular advantages with regard to treatment of picornaviruses. Related pharmaceutical compositions and methods of treating a picornavirus infection are also disclosed.
