Laimosemion ubim can be diagnosed from all other species of rivulids by its epipleural ribs branching at their-basal portion, and again at the sub-distal part of the dorsal branch formed by the sub-basal branching (vs. unbranched in all other rivulids, except for some species of Laimosemion with a single basal branching, but the branches formed are joint by an osseous flap. It also differs from all congeners by the possession of hypertrophied teeth on the anterior part of the outer row of the premaxillary and dentary in males (vs. teeth never hypertrophied) (Ref. 95522).

Description: longitudinal series of scales 29-30; transverse series of scales 6;