Between 28 January and 9 February 2003, c. 4000 heavily oiled seabirds washed ashore in The Netherlands, representing 21% of c. 20,000 casualties of the Tricolor oil spill recovered in northern France, Belgium and The Netherlands. Common Guillemot Uria aalge, Razorbill Alca torda and Black-legged Kittiwake Rissa tridactyla together represented 91% of the birds found and collected. Nearly 600 Common Guillemots and 267 Razorbills were examined in more detail and a large part of these birds were dissected. Autopsies revealed that the auks were in excellent condition when they died, indicating instant death through suffocation in oil. Of 440 Guillemots that could be aged, 76% were mature birds, with a sex ratio significantly different from equal (65% males). Of 262 Razorbills, 77% were adults and 62% were males. Biometrics suggested that the Guillemots belonged to the nominate subspecies, whereas the Razorbills were classified as A.t. islandica. Using the European cline in wing length, the Guillemots possibly originated from Scottish colonies (57°N), a finding that was supported by ringing recoveries (Scottish east coast). The total mortality caused by the Tricolor spill, 2-5 times the number of casualties recovered, may be estimated at 40 000-100 000 seabirds (25 000-62 500 Guillemots, 8000-20 000 Razorbills). With such a high proportion of mature birds in excellent pre-breeding condition being killed, an immediate effect on the breeding population is foreseen, rather than a diffuse and delayed effect if more immatures would have been killed. It is argued that effect of the Tricolor on seabirds would have been considerably less, had the salvage operation be postponed till summer. The Channel area is of the highest vulnerability to oil pollution only in winter (Dec-Mar).