菌丝体（英语：Mycelium(-a)，又称为菌丝球），指真菌的营养生长部分（相对于生殖生长部分的子实体），由许多分枝的菌丝组成。大块的菌丝体有时会被称为「shiro」，特别是仙女圈真菌的菌丝体。菌丝体可以存在土壤或许多其他基质中。一颗典型的孢子可萌发出一个同源的菌丝体，尚不能进行有性生殖，两个同源菌丝体进行融合并形成一个双核菌丝体后才能进行有性生殖，并产生子实体（例如蕈类）。菌丝体的大小可以小到肉眼难以观察，也可以十分庞大。

真菌透过菌丝体来从环境吸收水分与矿物质，也透过胞外消化吸收有机养分。首先菌丝释放酶到周围的基质中，将多糖、多肽等生物聚合物分解成单体，这些小分子养分随后透过促进性扩散或主动运输进入菌丝体中。

Mycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a homokaryotic mycelium, which cannot reproduce sexually; when two compatible homokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive: