Howlite, Ca2SiB509(OH)5, is monoclinic, P21/c,a = 12.78±0.03, b = 9.33 ± 0.02m c = 8.60 ± 0.02 Å,β = 104.83 ± 0.10“, Z= 4. Using the 1500 hkl intensities estimated visually from Weissenberg photographs, the structure was determined by locating the Ca atoms from a Patterson synthesis and placing the oxygen, boron and silicon atoms following study of a three-dimensional electron-density map based on the Ca signs. Least-squares refinement of the 861 data with |F|>0 reduced the conventional R-factor to 0.10. Study of infrared spectra had prompted the suggestion by Moenkc (1960) that boron occurred in both tetrahedral and triangular coordination in the structure. The suggestion is now verified because the structure contains colemanite-like chains with two tetrahedra and a triangle. These chains are located on the edges of anion slabs which contain spirals of silicate plus borate tetrahedra in the central region between chains. The slabs are tightly held together bv single and double chains of Ca coordination polyhedra. Three of the five hydroxvl ions are involved in hydrogen bonding. The absence of any observed cleavage is in accord with the structural character.