In 1999 a Black Tern Chlidonias niger ringing project was started in ‘Het Groene Hart’ (The Green Heart’) in the Provinces Utrecht and Zuid-Holland of The Netherlands. The aims are estimating of adult and juvenile mortality and dispersal. For this reason Black Terns are captured yearly in a more or less equal sample in a fixed area (Demmerik) as well as less systematically in adjacent areas. Adult tems are trapped at their nests as well as on night roosts. Although the project has just started, the first results are promising and interesting for publication and necessary for a sound discussion about disturbance. In total 111 adults and 51 chicks/juveniles were captured in the period 1999-2002. An interesting recapture was an adult captured on migration at the Belgium coast on 29 April recaptured twenty days later at the nest already breeding for eleven days. This means the bird had only nine days for migration, and starting the breeding process. Black Tems ringed as chick and recaptured as adults breeding had replaced on average 12.4 km (n=5) while adults recaptured as adults had replaced on average 400 m (n=6). This points high site tenacity in adults and higher dispersal distances in juveniles. The eldest tems were eleven and fourteen years. In the egg phase disturbance effects were minor as found in other studies. Eighty adults were captured on the nest with an average breeding period of 12.3 days. Clutch survival of these nests was 90.2% and egg loss was only 4.4%, both relative good figures. In the chick period disturbance is one of the most important factors for low breeding success of Black Terns in The Netherlands. Ringing can cause severe disturbance and consequent chick loss if not executed with greatest care. For this reason only chicks that were just almost able to fly were ringed. In total 46 chicks were ringed in this way and breeding success in ringing colonies (64.7% successful pairs, 0.9 young per pair; n=133) did not differ from breeding success in non-ringing colonies. Because adults seem to be less prone to be recaptured in consequent years, in 2002 a colour ring programme is started in one area.