The aim of this paper is to evaluate the long-term trend in breeding abundance of Black-headed Gull (Chroicocephalus ridibundus) at the Chomoutov lake, Czech Republic. In the period 1978-2014 (always between April 22 and May 10), we conducted 14 nest counts in the bird colony using the direct nest search method. In total, we counted 57,860 nests, from which 2,174 nests were found in the initial year 1978, the maximum number of nests (8,371) in 1993 and 3,304 nests in the last counting year 2014. The nests were located on the ground on an island elevated 2-3 m above the water level. In 1997, we found an unusually located nest on a nesting pad in a poplar tree (Populus sp.), 821 cm above the water level. In this paper, we point out the long-term decline in breeding abundance of Black-headed Gull in major breeding colonies in the Czech Republic, where the abundance declined by up to 95%, while numerous colonies have vanished completely. We discuss factors influencing the negative population trend of Black-headed Gull on the monitored site and in the entire Czech Republic.