Police chief Mir Zubair Mahmood said the blast in Quetta, the capital of Baluchistan province, set the bus on fire. He said another 19 people were injured.
Earlier, in a different town in the same province, militants destroyed a historic house associated with the country's founder, shooting dead a guard.
The province is home to Islamic militant movements as well as an ethnic nationalist insurgency.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? firstname.lastname@example.org