POWER OF ATTORNEY



The  undersigned  on his or her own behalf  and on behalf of his or her  estate,
heirs and assigns hereby  constitutes and appoints Susan S. Newton,  Christopher
R. Bohane,  Timothy M. Fagan,  Susan A. Pereira and Brian E. Langenfeld and each
of them singly as true and lawful  attorneys with full power to them and each of
them, to execute,  on behalf of the  undersigned,  in the capacity listed below,
any and all  statements or reports under Section 16 of the  Securities  Exchange
Act of 1934, as amended,  with respect to the beneficial ownership of reportable
securities  issued by MFS  California  Insured  Municipal  Fund, MFS High Income
Municipal  Trust, MFS InterMarket  Income Trust I, MFS Intermediate  High Income
Fund, MFS Investment  Grade  Municipal  Trust and MFS High Yield Municipal Trust
including, without limitation, all initial statements of beneficial ownership on
Form 3, all  statements  of changes of  beneficial  ownership  on Form 4 and all
annual  statements  of  beneficial  ownership  on Form 5, and any and all  other
documents  that  may be  required,  from  time to  time,  to be  filed  with the
Securities  and  Exchange  Commission,  to  execute  any and all  amendments  or
supplements  to any such  statements  or forms,  and to file the same,  with all
exhibits  thereto,  and  other  documents  in  connection  therewith,  with  the
Securities and Exchange  Commission,  granting to said attorney,  full power and
authority  to do so and  perform  each and  every act and  thing  requisite  and
necessary  to be done in and about the  premises,  as fully to all  intents  and
purposes as the undersigned  (or his or her estate,  heirs and assigns) might or
could do in person,  hereby  ratifying and confirming all that said attorneys or
any of them may lawfully do or cause to be done by virtue hereof.

IN WITNESS WHEREOF, the undersigned has set his or her hand on this ________ day
of June _, 2007.



JAMES O. YOST
James O. Yost