I've got a datasheet subform with a column 'InvVal' to which I've applied the custom format:
#,##0;-#,##0[Red]
which makes negative values appear in red.

This is Great, but I would prefer to make the entire row appear red if the value of InvVal is negative.

Is this possible??

if you make the fields background Transparent, you can change the background color of the details section to red. Other wise you will need to make each control's background color red. You will have to do this on the Form_Current Event. Evaluate if the number is Negative (>0).

NOTE: If you are not using Access 2000 this will change the look of all Records. Access 2000 allows for Record by record formating, the others don't.

I should have mentioned I'm using Access 97.

I saw on The Access Web a description of setting a single line of a continuous form to a different color. You might want to look at this:
http://www.mvps.org/access/forms/frm0047.htm

Chuck