long transaction with concurrent SELECT statements

Hi,

I have a transaction that updates several tables und runs a few minutes. 
During this transaction, another connection makes SELECTs from tables 
that are being modified by the first connection. Althaugh the second 
connection doesn't modify any rows, it has to wait until the first 
transaction is commited.
I didn't specify a concurrency mode and the first transaction didn't 
explicitly lock any rows or tables for update.

Do you have an idea what might be wrong here?

Greetings
Tim