Abstract: The crystal structure of ivermectin (H₂B_1a) was determined by X-ray diffraction. Ivermectin was found to be composed of more than 80% H₂B_1a and less than 20% H₂B_1b. The crystal of H₂B_1a belongs to the space group C 2 with a=40.736(6)A, b=9.295(3)A, c=14.966(3)A, β=106.93(1)°. After structure refinement using leastsquares method, the final R factor of H₂B_1a is 0.0658. The space configuration of H₂B_1a is a long chain along the Caxis. The molecular aggregates are extended into pairs of long chains parallel to each other, some distance apart. The distance between chains is the same, but their direction is opposite. This structure of H₂B_1a is similar to some biomacromolecules.