The defect structures of large single crystals of lithium formate monohydrate. HCOOLi · H2O (space group Pbn21), have been examined by X-ray topography. Two types of planar defects occur: a) Twin boundaries and twin lamellae parallel to (100). The twinning is produced by a rotation of 180° around the axis [100]. b) Defect planes parallel to (010), which could not fully be characterized. The crystals contain grown-in dislocation lines with Burgers vectors parallel to [100], [010], [001], 〈110〉, and probably also parallel to 〈011〉 and 〈012〉. Many of these lines have moved after crystal growth. Bundles of essentially straight dislocation lines with distinct but not always sharply defined preferred directions occur. These directions are in reasonable agreement with directions calculated by minimizing the elastic dislocation energy per unit growth length. Furthermore, dislocation half loops with a Burgers vector [001] produced by plastic deformation have been observed. Some relations between the relative growth velocities of two neighbouring growth faces and the geometry of the corresponding growth-sector boundary are discussed.