Saxitoxin (STX) contaminates seafood and freshwater catchments worldwide. Conjugation of STX with biotin would enable new biochemical methods to quantitate STX and its analogues as well as diversify its utility as a research tool. We conjugated biotin at the region of the toxin normally occupied by a carbamoyl and this conjugate could concurrently bind both avidin/streptavidin and saxiphilin. Increasing the length of the linker between biotin and the STX portion of the semisynthetic analogue increased potency of saxiphilin binding of the STX moiety.