Prymnesin-B1 is a chemical with the molecular formula C91H132ClNO34.

It is a member of the prymnesins, a class of ladder-frame polyether phycotoxins made by the alga Prymnesium parvum.

It is a so called "B-type" prymnesin, which differ in the number of backbone cycles when compared to A-type prymnesins like prymnesin-2.

[1] Prymnesins-B1 is formed of a large polyether polycyclic core with several conjugate double and triple bonds, chlorine and nitrogen heteroatoms and a single sugar moiety consisting of α-D-galactopyranose.

The backbone of B-type prymnesins like prymnesin-B1 is reportedly made by giant polyketide synthase enzymes dubbed the "PKZILLAs".