Zucchini

Zucchini (courgette in French) are a variety of vegetable marrow, a summer squash. They are tender and tasty when young, but most varieties are tasteless when large and overgrown. When buying zucchini, look for squash that is firm and heavy for its size. Do not wash until ready to use.  Peeling is not necessary and the skin is thin and very fragile.

Mild bitterness in zucchini, like that in related species like cucumbers, may be result from environmental factors such as high temperature, low moisture, low soil nutrients, etc. The bitterness is caused by compounds called cucurbitacins.
There is also a rare condition which can cause extreme bitterness in zucchini. A compound called Cucurbitacin E is found in wild species of squash, but is extremely rare in cultivated species. Very, very rarely a mutant plant or a chance cross with a wild species may result in cultivated plants with Cucurbitacin E. There will be an acrid smell when you cut the zucchini, and just touching the flesh to your tongue will have an extremely unpleasant bitter taste. Do Not Eat such zucchini. They may cause cramps, diarrhea and even collapse. If you know where the seeds came from, notify your local Agricultural Extension service.

Remember, this is not the normal bitterness which occurs occasionally with zucchini and related species. This is a very unpleasant and very bitter taste and is an extremely rare occurrence.