Fecundity and growth allocation to reproduction were investigated in stream-dwelling threespine Gasterosteus aculeatus L. (leiurus) and ninespine Pungitius pungitius (L.) sticklebacks of St. Ippollitts Brook, Hertfordshire (England), between August 1993 and October 1994. We compared oocyte size distributions as well as the relative fecundity of females containing mature oocytes, compared growth allocation to gonadal growth and overall reproductive allocation from estimates of total egg supply, the number of reserve oocytes eventually spawned, the proportion of reserve oocytes not spawned, the total number of oocytes spawned, and the number of egg clutches per season. Notable differences in reproductive strategy were observed between the two species. Ninespine sticklebacks were smaller in size than threespine, but compensated for this through a smaller size at first maturity, an almost equal seasonal fecundity (number of spawned oocytes) accompanied by a higher number of unspawned oocytes (i.e. maintenance of a low mature-to-total oocyte ratio), an extended spawning period, a higher number of clutches, and an overall lower reproductive allocation.