This study was carried out in Sokoto Metropolis to isolate and identify fungi associated with the deterioration of sweet orange fruits. A total of one hundred samples of fresh sweet Oranges (Citrus sinensis L) were used. First, a total of seventy samples were obtained from the three selected marketing centres in Sokoto metropolis (Central market, Kasuwar daji and Old market respectively), and transported immediately to mycology laboratory Usmanu Danfodio University, Sokoto for analysis. The oranges were kept under room temperature and observed after two weeks for spoilage. Each of the orange was cut and the liquid content inoculated on potato Dextrose Agar and incubated at 25 0 c and observed for 3-28 days after which the different colonies obtained were identified using the slide culture technique. The fungal analysis shows that Apergillus fumigatus, Apergillus niger, Aspergillus flavus and Rhizopus stolonifer are associated with the spoilt sweet orange fruit (Citrus sinensis) with frequencies of occurrence of 22%, 17 %, 25 % 36% respectively. Fresh sweet orange fruits (thirty samples: ten each from the three markets respectively) were obtained, and the isolated culture for each of the identified fungi specie inoculated on each of the freshly purchased oranges and incubated to check for their spoilage ability. The result shows that R. stolonifer and A. flavus were the most active with rots diameter of 45 mm and 35 mm, respectively. And the least active fungus was A. niger having a rots diameter of 25mm.