>>>>Is it only this one textbox on that form? Also, why do you reference company in your code and not zip?
>>>
>>>There are dozens of textboxes on the form. company is the datatable. zip is the column.
>>
>>I mean - is it the only one misbehaving on that form?
>
>No. I used it as an example. Others do similar things but some work.
>The problem seems to occur as the column becomes larger.
>For example, a char(2) column with a maxlength of 2 works fine.

Ok, I don't know - I don't use WinForms.