>>>
>>>thanks, but no, clientType is never null.
>>
>>Assuming clientType isn't null, something else in that code is failing (probably because of a null).
>
>Thanks again (and Mike too).
>
>In the meantime I had changed the code to this:
>
>

        private void SetClientTypeDisplay(string clientType)
>        {
>            this.pnlCompany.Visible = (clientType.Trim() == "Company");
>

>
>and that works, so I guess I'll never work out what the problem was (unless I get a chance to put it back to the original and step through it).

Whatever the problem was I'm afraid that change in code won't fix it - it will still be waiting to bite you....
If the error occurred in the snippet you posted then either clientType or pnlCompany was null when it raised the exception....