find records with only numbers

[Ed Reed]

I hope someone can give me a suggestion on this.
 
I'd like to find records in a table where a specific field only contains a 
number. 
 
For example,
Select Field1 as f
from table1 as t
where lcase(t.f) not like in 
('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z')
 
This obviously doesn't work or I wouldn't be asking the question. In this 
example field1 is a varchar(25) field and it is normally appropriate for it to 
contain data that has numeric characters and alpha characters. Examples of 
valid data in this field are '456987','142154','200145C1','954xxx','H 1231','My 
Test', ......etc. In my query above I'm trying to find the records where there 
is only a six digit numeric value and no others.
 
Thanks for any advice