Mouth widths of eels in three Dutch lakes were compared relative to feeding conditions over two years. Average mouth width varied by season and year and per lake. Of two groups of feeding specialists distinguished in each population chironomid feeders showed a smaller mouth width compared to fish feeders of the same body length. Proportions of these specialists differed between years, lakes, and seasons and changed in response to changing feeding conditions. The shift to fish feeding occurred when chironomid biomass decreased and young fish recruited. The shift was greatest in the year when chironomids collapsed completely and in the lakes with high eel densities. Mature males were narrow-headed and left the lakes at sizes between 30 and 40 cm. Larger eels were always broad-headed females. Implications for foraging efficiency and population structure are discussed.