What is the default transaction isolation level for SQL Server with ADO.NET?

READ COMMITTED is the default isolation level for the Microsoft SQL Server Database Engine.

Source:

• Customizing Transaction Isolation Level @ MSDN.

Here is how it compares to other isolation levels:

• Transaction Isolation Levels @ MSDN.

The MSDN documentation for SqlConnection.BeginTransaction() also states Read committed

… To reset the isolation level to the default (READ COMMITTED) …

• MSDN article