Three groups of males were examined for the study of Mycoplasma in the aetiology of Non-specific Urethritis. From a group of 120 men suffering from N.S.U. (group I) have been isolated from the genital tract "large-colony" and T-strain mycoplasma in proportion 22.5% and 68.3% respectively. Socially this group represent the lower classes of the population. From another group of 165 male students suffering also from N.S.U. (group II) have been isolated from the genital tract "large-colony" and T-strain mycoplasma in proportion 25.4% and 58.1% respectively. This group was of higher social and hygienic standards. In contrast in a control group of 70 medical students without urethritis the range was for the "large-colony" 8.5% only and for the T-strain 21.4%. Socially, this group corresponded closely to the Group II. In this study a significant association of T-strain mycoplasma with N.S.U. has been found. Urethral specimens were obtained from all the men by the same method. No statistically significant difference in the isolation rate of Mycoplasma was seen, between Group I which represents a relatively low social and hygienic level and Group II which represents higher social and hygienic standards. It is interesting to note that the percentage of control healthy group (group III) yielding T-strain mycoplasma was lower and no similar to that recorded in previous investigations.