Algeria is the North African country with the largest number of alien weed species. The analy- sis of Oranie's weed flora showed 29 species of alien species which represents 1.62% from the 1,780 species of the whole spontaneous flora of the oranian phytogeographic territory and 6.82% of the local weed flora. Almost 40% of these species were native to the American con- tinent and about 45% came from several tropical regions. The most representative life-history traits of these alien species compared to native ones were: annual cycle (79% versus 76%), summer germination (83% versus 15%), non-specialized dissemination strategies, but related with anthropic activities (72% versus 47%), self-pollination (75%), and the high proportion of C4 species (45% versus 4%). The agricultural biotopes mainly affected are summer-irrigated crops with short rotation and important nitrogenous fertilizers assignment. There are very few such habitats in Oranie, and few summer annuals in Algeria's flora. The success of alien weeds seems to result from suitability between environmental conditions of a newly created habitat (summer temperature, humidity, important nitrogen rate, frequent perturbations, short rotations, and ecological niches available) and species characteristics (annual type, summer germination, photosynthetic pathway in C4). Dispersal strategy and fecundation types seemed not to be of much importance to characterize these alien species.