The cereal that Michelle Prevezer bought was Nestlé's Raisins, Oats & More. But the "More" turned out to be more than she bargained for.
Ms Prevezer, 44, a mother of one from Borehamwood, Hertfordshire, had munched her way through about a third of the cereal when she suspected she might have bitten into something she should not have.
An independent analysis showed that the intruders were oryzaephilus surinamensis - saw-toothed grain beetles.
"I thought there were what looked like a lot of raisin stalks - but then one moved," said Ms Prevezer, whose 15-year-old son attends Yavneh College. "I went to pick it up, and it jumped on to a table. There were loads of them crawling about in the bottom of the box.
"I felt ill at the thought that I had probably eaten some of these insects."
The following day she called Nestlé and sent the box and its contents back to them for analysis.
"They took a month to reply and, to add insult to injury, they sent me a cheque for a pathetic £20."
A Nestlé Cereals spokesperson said: "We sent Ms Prevezer a cheque as a gesture of goodwill, apologised for any inconvenience and hope all her future purchases will be satisfactory."