Oocytes from prepubertal (PRE) or postpubertal (POST) pigs are used in, for example, somatic cell nuclear transfer and in vitro fertilization. Here we describe mitochondrial dynamics in pig oocytes of different sizes before and after in vitro maturation (IVM), isolated from PRE or POST animals. In PRE oocytes, inside-zona pellucida diameter was measured before and after IVM (μm; small: ≤110, medium: >110, large: ≥120) and used for evaluation of (1) mitochondrial numbers before maturation and (2) mitochondrial morphology and location before and after maturation in comparison with POST oocytes. Oocytes were processed for transmission electron microscopy (Acta Anat. 129:12). For assessment of mitochondrial numbers, paired dissector sections were collected at uniform intervals throughout the oocyte, and in each set of dissector sections a known area fraction was sampled for mitochondrial counting in physical dissectors (J. Microsc. 134:127). Total number of mitochondria was calculated, and oocyte volume was estimated by Cavalieri estimator (J. Microsc. 147:229). Data were analysed by ANOVA. Mitochondrial morphology was classified as elongated, round, shell-like, or compartmentalized; mitochondrial cristae as transverse or peripheral; and mitochondrial location as cortical, subcortical, or central. Before IVM, small PRE presented elongated and round mitochondria with transverse cristae; medium and large PRE presented round mitochondria with peripheral and transverse cristae; POST presented round mitochondria with peripheral cristae in all cases. After IVM, small and medium PRE had round mitochondria with peripheral cristae; medium PRE and POST had shell-like mitochondria with peripheral cristae; large PRE had compartmentalized mitochondria with peripheral cristae. Before IVM, small PRE displayed cortical mitochondrial location, whereas the location in other groups was cortical and central. After IVM, mitochondria were located centrally in some large PRE and in all POST. Mitochondrial number increased during oocyte growth proportional to the increase in oocyte volume (Table 1). Shell-like and compartmentalized mitochondria indicate (1) dividing mitochondria (increasing mitochondrial numbers during maturation), or (2) apoptosis-related mitochondrial fission (compromised oocytes after maturation). After IVM, mitochondria seemed to reach the final central position most consistently in POST. These differences may partly explain the higher developmental competence in larger PRE and POST oocytes.