Mephibosheth,
The A# is the alien registration number, assigned by USCIS.
The item is part of the previous visits to or residences in the USA question - yes? If one has previously resided in the USA then one probably does have an A#, if one has only visited or has not been in the USA then one probably does not. Although, now that the USCIS does get involved in the processing of I-130 petitions submitted via DCF, one may have been assigned an A#.
If the visa applicant doesn't know if they have an A# then leave the space empty - note that the number is asked for "if known".
Yodrak
QUOTE(Mephibosheth @ Jul 9 2007, 04:13 PM)

On page two of the DS 230 it asks for the ''A'' NO. if known... is that the Admission number???? Or the Alien Number or some sort of other number??? How does one know these things?