As of fall 1992, ISSN centers (including NSDP) began to implement a new
policy regarding assignment of ISSN to publications in various physical
formats.  The policy is as follows:  separate ISSN are assigned to serials
issued in different physical formats UNLESS the different format represents
simply a reproduction.  So, for example, a different ISSN is assigned to
a print journal and its online counterpart.  Also, a different ISSN is
assigned to a print journal and its CD-ROM counterpart.  However, a print
journal and a microform reproduction (such as a reproduction made as part
of a preservation microfilming project, or, by extension one made by a
commerical producer such as University Microfilms) will be assigned the
same ISSN.

This policy has not been acted upon retrospectively, so there will be
some serials for which these separate ISSN assignments have not been made.

I'll be happy to TRY (some of these distinctions are not easy ones) to
answer questions about ISSN for multiple physical formats that list readers
might direct to me.

Regina Reynolds
Head, National Serials Data Program
Library of Congress
(202) 707-6379
rrey@seq1.loc.gov