Exhibit 31.1
                       CERTIFICATIONS

I, Robert P. Johnson, certify that:

1. I have reviewed this quarterly report on Form 10-Q of AEI
Income & Growth Fund XXI Limited Partnership;

2.  Based on my knowledge, this report does not contain  any
untrue  statement  of a material fact or  omit  to  state  a
material  fact  necessary to make the  statements  made,  in
light of the circumstances under which such statements  were
made,  not misleading with respect to the period covered  by
this report;

3.  Based  on  my  knowledge; the financial statements,  and
other  financial information included in this report, fairly
present  in  all material respects the financial  condition,
results  of  operations and cash flows of the registrant  as
of, and for, the periods presented in this report;

4.  The  registrant's other certifying officers  and  I  are
responsible  for  establishing  and  maintaining  disclosure
controls  and procedures (as defined in Exchange  Act  Rules
13a-15(e) and 15d-15(e)) for the registrant and have;

       a)  designed such disclosure controls and procedures,
or  caused  such  disclosure controls and procedures  to  be
designed  under  our  supervision, to ensure  that  material
information  relating  to  the  registrant,  including   its
consolidated  subsidiaries, is made known to  us  by  others
within  those  entities, particularly during the  period  in
which this report is being prepared;

       b)   evaluated the effectiveness of the  registrant's
disclosure  controls and procedures and  presented  in  this
report  our  conclusions  about  the  effectiveness  of  the
disclosure  controls and procedures, as of the  end  of  the
period covered by this report based on such evaluation; and

       c)   disclosed  in  this report  any  change  in  the
registrant's internal control over financial reporting  that
occurred during the registrant's most recent fiscal  quarter
(the  registrant's fourth fiscal quarter in the case  of  an
annual   report)  that  has  materially  affected,   or   is
reasonably  likely  to materially affect,  the  registrant's
internal control over financial reporting;

5.  The  registrant's other certifying officers and  I  have
disclosed, based on our most recent evaluation, of  internal
control   over  financial  reporting,  to  the  registrant's
auditors  and the audit committee of registrant's  board  of
directors (or persons performing the equivalent functions):

        a)    all   significant  deficiencies  and  material
weaknesses  in the design or operation of internal  controls
over  financial  reporting which are  reasonably  likely  to
adversely   affect  the  registrant's  ability  to   record,
process,  summarize  and  report  financial  data  and  have
identified  for  the  registrant's  auditors  any   material
weaknesses in internal control over financial reporting; and

       b)  any fraud, whether or not material, that involves
management or other employees who have a significant role in
the registrant's internal controls.




Dated:  August 8, 2008        /s/ Robert P Johnson
                                  Robert P. Johnson, President
                                  AEI Fund Management  XXI, Inc.
                                  Managing General Partner