Yes. The framework allows you to use any ADSI provider including LDAP, NDS
etc.
From MSDN:
System.DirectoryServices - consists of classes that provide easy access to
the Active Directory from managed code. The classes in this namespace can be
used with any of the Active Directory service providers. The current
providers are: Internet Information Services (IIS), Lightweight Directory
Access Protocol (LDAP), Novell NetWare Directory Service (NDS), and Windows
NT. For more information about the Active Directory, see Directory Services.
Richard