Welsh Water has written to 9,000 customers in north Wales telling them they no longer need to boil their drinking water.
The "all clear" notice came after the supply to many houses was switched from the Cwellyn reservoir in Snowdonia.
The treatment works there is under investigation after an outbreak of the water-borne bug cryptosporidium.
The number of illness cases stands at 186. Meanwhile, about 50,000 people must continue to boil their water.
Last month, health officials advised 36,000 households - about 70,000 people - in north Wales to boil or buy water to protect against the stomach bug.
The boiling order for 9,000 customers - just under 20,000 people - has now been lifted, but officials warned on Friday that customers who do not receive a notice should continue to boil their water until 9 January.
Residents wishing to find out whether their home's supply is affected can check their postcode against a list held by Dwr Cymru Welsh Water at any time by ringing 0800 052 0130.
NHS Direct is operating two help lines on 0845 850 9850 and 0845 600 6086 between 0900 GMT and 1800 GMT on weekdays, and 0900 GMT and 1700 GMT at weekends.
Welsh Water spokesman Wynford Emmanuel told BBC Radio Wales that he would not speculate about the lifting of the ban for other customers.
"The principal thing we have to do is protect public health," he said.
"The crucial thing is that we find out what the cause of the outbreak is."
Some of the households informed about the lifting of the water restrictions, had lived outside the area supplied by Cwellyn and had been told to boil water needlessly.
Apologising to those customer Welsh Water's Tim Masters said: "Our initial mailing of a letter to customers asking them to boil water aimed to ensure that we covered everyone affected.
"We are aware that letters were sent to some customers who are not supplied by the Cwellyn works.
"To avoid undue delay in giving customers important information, we decided to err on the side of caution to ensure that we reached all those who needed the notification.
Customers should receive a letter giving them the all clear within the next few days.
The National Public Health Service said the investigation into the cause of the outbreak was continuing. On Friday, the number of cases of cryptosporidium rose to 186, up 12 on Thursday.
The areas where households no longer need to boil water are:
Bethel, Bodorgan, Brynsiencyn, Cerrigceinwen, Dwyran, Glanrafon, Hermon, Llangadwaladr, Llangaffo, Llangefni, Llangristiolus, Malltraeth, Niwbwrch, Pentre Berw, Trefdraeth.
Bethesda, Brynrefail, Clwt-y-Bont, Cwm-y-Glo, Deiniolen, Dinorwic, Fachwen, Gallt-y-Foel, Gerlan, Glasinfryn, Llandegai Ind Est, Llandegai, Llanllechid, Mynydd Llandegai, Penisarwaen, Rachub, Rhiwlas, Tal-y-Bont, Tanysgafell, Tregarth.
Customers living in the following area must continue to boil their drinking water:
Beaumaris, Ceint, Gaerwen, Llandaniel, Llandegfan, Llanfaes, Llanfairpwllgwngyll, Llangoed, Menai Bridge, Penmynydd.
Bangor, Beddgelert, Bethania, Bethel, Betws Garmon, Bontnewydd, Caeathro, Caernarfon, Carmel, Dinas, Ffridd Uchaf, Groeslon, Llanberis, Llandwrog, Llanfaglan, Llanrug, Llanwnda, Nantmor, Pont Aberglaslyn, Penyffridd, Plas Gwynant, Pont-Rug, Rhosgadfan, Rhostryfan, Rhyd-Ddu, Seion, Vaynol Hall, Waunfawr, Y Felinheli.