why does explain tell me I'm using a seq scan?

I have indexed two columns in a table.  Can somebody explain to me why
the first query below uses an Index Scan while the second uses a Seq
Scan?

Many TIA!
Mark


planb=# \d abcs
                                Table "public.abcs"
   Column   |          Type          |                   Modifiers
-----------+------------------------+-----------------------------------------------
  abcid   | integer                | not null default nextval('abcid_seq'::text)
  type      | character varying(255) |
  versionof | integer                |
Indexes: abcs_pkey primary key btree (abcid),
          abcs_versionof btree (versionof)

planb=# explain select type from abcs where abcid = 6339;
                                  QUERY PLAN
----------------------------------------------------------------------------
  Index Scan using abcs_pkey on abcs  (cost=0.00..6.01 rows=1 width=145)
    Index Cond: (abcid = 6339)
(2 rows)

planb=# explain select type from abcs where versionof = 6339;
                            QUERY PLAN
----------------------------------------------------------------
  Seq Scan on abcs  (cost=0.00..59182.10 rows=16137 width=145)
    Filter: (versionof = 6339)
(2 rows)