Reproductive features of Big-Scale sand smelt, Atherina boyeri (Risso, 1810), which is an exotic fish species in Lake Egirdir (Turkey) were studied between March 2006 and August 2007 using 1433 individuals. According to 18 month GSI results, egg maturation starts in March of both 2006 and 2007 (2.74 and 1.67), and peaking in late June (9.16 and 13.44), decreases to its minimum level in July (4.61 and 1.41); spawning occurs late July. First mature length of females (Lm) was determined as 45.93 mm and relative fecundity 29.05 eggs g(-1). Relationships of total length (TL), weigth (W) and gonad weigth (GW) with fecundity were calculated respectively as F=181.63xGW(0.5478), F=0.104xTL(1.5439) and F=55.443xW(0.4082); mean egg diameter was 1.03 +/- 0.016 mm.