The Spiny lanternfish, Myctophum spinosum (Steindachner, 1867) (Family: Myctophidae) is a bathypelagic, oeanodromous species that exhibits diurnal vertical migration and is widely distributed in tropical and sub-tropical waters of the Indian, Pacific, Indo-Pacific and Atlantic Oceans. M. spinosum is a common species in the bycatch of deep sea trawlers, operating off southwest coast of India. Length-weight relationship provides a means for calculating weight and biomass when only length measurements are available and has been used as a character for the differentiation of taxonomic units and this relationship is seen to change with various developmental events in life. In the present study the length-weight relationship of M. spinosum, caught off south-west coast of India, have been estimated as: W=0.0129 L2.7372 , for both males and females (pooled data); W=0.0212 L2.4802 , for males; and W= 0.0077 L2.9987, for females. The total length of M. spinosum ranged from 64 to 90 mm, with a mean length of 72 mm and the weight of the fishes ranged from 2.1 to 6.2 g. Female fishes exhibited isometric growth (p value = 0.625, df = 50) and male fishes exhibited negative allometry (p value = 0.007, df=61). There was no significant difference in the slopes (p>0.05) between male and female fish (ANCOVA).