Background. Monkey goby is one of many Neogobius species that have invaded European waters in recent decades. Little interest has been paid to this species up till now, probably due to its lower rate of invasiveness compared to some other goby species, e.g., round goby. As with any non-native species, however, it represents a potential danger for native fauna. The aim of this study is to fill in the missing data concerning monkey goby biology and population structure in its native area of distribution, thereby providing a baseline for future comparative studies in non-native distribution areas. Materials and Methods. Samples of monkey gobies, Neogobius fluviatilis (Pallas, 1814), were obtained seasonally from the lower, Bulgarian stretch of the Danube River during 2006. The fish collected were sexed, measured, and weighed. After that the gonads were removed and weighed. These data were used to calculate sex ratio, length-frequency distribution, condition coefficient, and gonadosomatic index. Dissected ovaries were used for histological analysis of oocyte distribution. Results. The monkey goby population displayed a relatively balanced sex ratio, while length-frequency distribution demonstrated two- (April), and later three (October), clearly visible peaks, representing three age classes. The condition factor of both males and females was influenced by season. We were able to estimate the starting time and the duration of the spawning season using both gonadosomatic index and ovarian histological analysis. Conclusion. Population of monkey goby in the Bulgarian Danube is an example of a stable native population defined by balanced sex ratio, three-year life span, and recruitment represented by strong cohort of 0+ fish, some of them maturing in their second year of life.
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