In 2007-2009 in the vicinity of Bydgoszcz 6 extensively cultivated fields of fodder galega (Galega orientalis Lam.), established 4-21 years ago, protecting light soils as fallow, were converted into seed plantations. Detailed observations and measurements of the values of biological seed yields and yield-forming factors were carried out. The aim was to estimate the values and dynamics of yield factors and the effect of periodically cultivated seed plantations of galega on soil fertility. Biological yield of galega seeds was from about 7 g*m-2 to over 113 g*m-2 and was positively correlated with the age of plantations (r=+0,406). Seed yield was also positively correlated with yield factors, in order with the number of pods per shoot (r=+0,614), the density of fruit-bearing shoots (r=+0,546), the number of seeds per pod (r=+0,388) and seed size (r=+0,235). Biological seed yield was limited by the density of unproductive shoots (r= -0,377), whose development was favoured by rainfalls in June. The fertility index of flowers was on average 40%. During 3 years of the study the soil fertility under fodder galega cultivated temporarily for seeds did not decrease. The effect of changes in soil fertility on seed yield was small.