* Protests to Dar over mistreatment of Dutch investor

* Row threatens $41.93 mln in budgetary support

DAR ES SALAAM, July 6 (Reuters) - Netherlands has written to Tanzania to protest over the treatment of a Dutch investor and could freeze its aid to the east African nation, local media and a Dutch embassy official said on Monday.

Late last week, Dutch media said Bert Koenders, minister for development cooperation had written to the lower house of parliament to say Tanzania was unreliable for foreign investors, after an unnamed Dutch businessman had been mistreated.

No details of the alleged mistreatment have been officially released by either side.

The Dutch Embassy in Dar es Salaam confirmed the government had sent the letter to Tanzania as well. It said the aid in question was 30 million euros ($41.93 million) direct support for the country's 2009/10 (July-June) fiscal budget.

"We can't give any comment at the moment because we sent a letter to the government. It's awaiting an answer from the Tanzanian government and it's only about general budget support," embassy spokeswoman Monique Korzelius told Reuters.

Local media quoted Finance Minister Mustafa Mkulo confirming receipt of the letter.

"It is true that the (Tanzanian) government has received a letter, regarding the suspension of aid by the Netherlands, but we are yet to give them an official response," Mkulo was quoted as saying by the privately-owned ThisDay newspaper.

Tanzania is among Africa's largest per capita aid recipients, with 33 percent of its 2009/10 budget of 9.51 trillion shillings ($7.26 billion) expected to come from external sources.

Some donors have expressed concern at the slow pace of reforms, including improvement of the business climate.

Besides the budgetary support, the Netherlands will give Tanzania another 51 million euros to boost healthcare and governance, in the 2009/10 fiscal year. (Reporting by George Obulutsa; Editing by Jon Boyle)

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