Problem installing with apt-get

[Reinhard Hnat]

I have an Ubuntu Server 8.04 and tried to install tomcat6 with the 
following result:

root at Mendelsson:/etc# apt-get install tomcat6
Paketlisten werden gelesen... Fertig
Abhängigkeitsbaum wird aufgebaut
Reading state information... Fertig
E: Konnte Paket tomcat6 nicht finden   (means "could not find packet 
tomcat6")

Installing for example Java with the same procedure works perfectly. 
What could be the cause?

Thanks in advance
R.Hnat