Move the code to the After Update event of the Salad Type Combo Box.

Thanks Kevin.
I did that but now get a runtime error 3075

You will probably have to refer to the name of your Combo Box, not ProductsID.
PricePerKg = Nz(Dlookup("PricePerKgWholesale","ProductsT", "ProductsID=" & YourComboName),0)

Replace YourComboName with the name of your Combo Box.
Also, check spelling. Is it ProductsID or ProductID?

Thanks Kevin, now it works.  Why use the Nz function? In VBA can I not use a a value in field A to update field C by using an After Update Sub in field B?

I added the Nz because if the price isn't found in the table for that product you will get an Invalid Use of Null Error.

No matter what I do, I always get the PricePerKgRetail. I have tried different ways as shown below.

If Wholesale Then
         PricePerKg = Nz(DLookup("PricePerKgWholesale", "ProductsT", "ProductsID=" & SaladCombo.Column(0)))
        Else
            PricePerKg = Nz(DLookup("PricePerKgRetail", "ProductsT", "ProductsID=" & SaladCombo.Column(0)))
        End If
_____________________________________________________________
If Wholesale Then
         PricePerKg = Nz(DLookup("PricePerKgWholesale", "ProductsT", "ProductsID=" & SaladCombo), 0)
        Else
            PricePerKg = Nz(DLookup("PricePerKgRetail", "ProductsT", "ProductsID=" & SaladCombo), 0)
        End If
______________________________________________________________
If Wholesale Then
         PricePerKg = Nz(DLookup("PricePerKgWholesale", "ProductsT", "ProductsID=" & SaladCombo), 0)
End If

If Wholesale = False Then
            PricePerKg = Nz(DLookup("PricePerKgRetail", "ProductsT", "ProductsID=" & SaladCombo), 0)
  End If

PricePerKgWholesale is 70, PricePerKgRetail is 120 (Thai Baht)

Your Check Box if on the parent form, use If Parent.Wholesale Then

Hi Kevin, that worked.  I had not seen the Parent.field yet.  I don't think Richard used it anywhere I've seen so far.