A short excursion was made to the western part of Mauritania to investigate increasing populations of the desert locust, Schistocerca gregaria, in October 2009. The size of locust populations observed ranged from <1 to >20 individuals per 25 m2. Adults from low- and high-density populations were similar in morphometric ratios of F/C and E/F (F, hind femur length; C, maximum head width; E, fore wing length) and the ratios were similar to those reported for solitarious forms. However, locusts from high-density populations were mostly yellow, while those from low-density populations were brown or whitish. The former were observed ovipositing in groups. These observations, together with the fact that some of the females from high-density populations had ovarian eggs significantly larger than those found in females from low-density populations, suggested that gregarization might have occurred after adult emergence. Aggregations of nymphs were observed at many sites, and nymphs with black patterns typically observed in transient and gregarious populations were found together with those with solitarious body coloration, i.e. green or brown. A prompt action of locust control by National Locust Control Center was likely to have contributed to prevent further upsurges in the locust populations in the following seasons.