The local custom is to call what are known in Spanish as tapas by the Basque name, Pintxos, probably much more elaborated than their Spanish cousins due to the Basque love of gastronomy.
Fri and Sat night are most popular time for eating pintxos--as it is for going to bars.
The Aritz in the Calle Zugastinobia 4 was once one of the best Pintxos bars in Bilbao, quite out of the way (almost hidden really in a very small, one way street away from the main thoroughfares). Ask for it and you will be given directions. It is worth the walk since they have won the local Pintxo contests for ages. Closed as of Feb. 2008 and as the summer of 2008 reopened with what appears to be new management with less emphasis on its prize-winning pintxos.
It's worth going to this street though because there are dozens other pintxos bars.
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.Based on a work at Wikitravel.org & Traveldudes.org.