Can you respond to your own fetchland activation by casting an instant?

When you activate a fetchland (e.g., Flooded Strand), its ability goes on the stack. Under CR 116.3c, after any player puts an ability on the stack, all players get priority in turn order before that ability resolves. Crucially, the active player receives priority first — meaning you get priority back immediately after your own activation.

Because you have priority with the fetch ability on the stack, you may cast any instant or activate any instant-speed ability before the fetch ability resolves. This is standard stack behavior described in CR 116.1: a player may cast instants or activate abilities whenever they have priority.

Once you are done acting (or choose to pass), opponents also receive priority in turn order. Only after all players pass priority in succession does the top object on the stack resolve (CR 116.4).

Concrete example: You activate Windswept Heath. With the fetch ability on the stack, you cast Brainstorm (an instant). Brainstorm resolves first, letting you arrange your library top. Then Windswept Heath's ability resolves and you search for a Forest — potentially shuffling away cards you put back with Brainstorm. This is a well-known interaction that works exactly because you have priority after your own activation.

So yes, responding to your own fetchland with an instant is always legal as long as you have priority and the card/ability is otherwise playable at instant speed.