Rose shrimp Parapenaeus longirostris(Lucas, 1846),  is one of the main target species of the trawl fleet working  in the western Moroccan Mediterranean ,and an important species of crustacean landings in M'diq port . Despite that, studies on biological characteristics of shrimp in this region are scarce. Hence, this study was aimed to evaluate the length-weight relationships of P. longirostris. Specimens were collected from the coastal trawlers in a monthly sampling over a period of twelve month between April 2017 and April 2018  at M'diq port,  western Moroccan Mediterranean, and the length-weight relationship of the shrimp was estimated using linear regression. Results from the present study indicate that the equations of total length TL, carapace length CL, and weight were expressed for females, males, and pooled sexes.  The length-weight relationship was determined as W = 0,0000119 × TL2,834,   r2=0.932 for total length, and W = 0,00340 × Lc2,312 , r2 = 0.978 for carapace length for pooled sexes. The length-weight relationship showed a negative allometric growth pattern, and the growth exponent (b) ranged from 2.296 to 2.834. The regression coefficient of females, males, and pooled sex were significantly different from isometric growth at P < 0.05.