ABSTRACT. – Posidonia oceanica (L.) Delile (1813) flowering, and particularly fruiting, is a rarely observed phenomenon in the Algerian coast. During two spaced surveys, inflorescences were observed in wrecks at Stidia (coastal area of Mostaganem, Algeria). Inflorescences from the first prospecting (November 6, 2017) bear only flowers. Each inflorescence carries between one and four spikelets, with an axis length ranging between 24 mm and 27 mm; the majority of the examined spikelets carries two to three flowers. The latter are 56.2 % to 78.6 % hermaphrodite and 21.4 % to 43.8 % are male. The inflorescences of the second prospecting (January 8, 2018) bear aborted fruits and ovaries. Each inflorescence carries between two and four spikelets, with an axis length ranging between 20 mm and 33 mm. The spikelets of the second prospecting carry a single fruit, sometimes two, and some do not carry any. The fruit length is between 7 mm and 15 mm. The aim of this work is to provide the first data collected on Posidonia oceanica flowering and fruiting phenomenon on the Algerian west coast (Mostaganem), offering thus the possibility to describe the morphology of the inflorescences and fruits. The collected data show some similarities to those obtained in different regions of the Mediterranean Sea.