The effect of the alkaline treatment on the yield of the extraction and physical properties of agar from Gracilaria verrucosa collected in Santa Veronica (Atlantico, Colombia) was investigated. The concentrations of sodium hydroxide solutions during the alkaline treatment were 3, 5, 7 and 10 % (w/v), while temperature was maintained in the range of 80-85 oC. The agar yield, gel strength, melting and gelification temperatures were determined. Although the maximum yield was obtained with 10 % sodium hydroxide solution, the maximum gel strength was obtained at 3 %. The agar-agar was used as a base of nutritive agar for 23 bacterial cultures and the productivity was measured using the ecometric method. From the bacterial strains tested, Salmonella Enteritidis, Salmonella Typhimurium, Salmonella Cholerasuis and Morganella morganii did not grow in the test or commercial media. The measures of relative growth index showed that nutritive agar from G. verrucosa have the same or better properties than the commercial ones.